Add FL/win32_target.h and fix pen support build on Windows

Part 1: the new file FL/win32_target.h simplifies setting minimal
  Windows version macros to support building with minimal Windows
  version symbols, such as for Windows 2000 or Windows 8, rather
  than hardcoding weird C/C++ macro code to define the symbols.

Part 2: apply this everywhere where WINVER and _WIN32_WINNT needed
  to be set manually.

Part 3: fix pen support builds on Windows by setting the correct
  minimal Windows version in the concerned source files.

Part 4: fix CMake compilation test for pen support: use a symbol
  that's really used in the sources.

CMake/pen-support.c

@@ -21,21 +21,11 @@
 
 /* We require Windows 8 or later features for Pen/Tablet support */
 
-# if !defined(WINVER) || (WINVER < 0x0602)
-#  ifdef WINVER
-#   undef WINVER
-#  endif
-#  define WINVER 0x0602
-# endif
-# if !defined(_WIN32_WINNT) || (_WIN32_WINNT < 0x0602)
-#  ifdef _WIN32_WINNT
-#   undef _WIN32_WINNT
-#  endif
-#  define _WIN32_WINNT 0x0602
-# endif
+#define FL_WIN32_TARGET_VERSION 0x0602
+#include "../FL/win32_target.h"
 
 #include <windows.h>
 
 int main() {
-    return POINTER_CHANGE_FIRSTBUTTON_DOWN; /* required symbol */
+    return POINTER_FLAG_SECONDBUTTON; /* required symbol */
 }

FL/win32_target.h

@@ -0,0 +1,81 @@
+/*
+ *  Define min. Windows version macros for the Fast Light Tool Kit (FLTK).
+ *
+ * Copyright 1998-2026 by Bill Spitzak and others.
+ *
+ * This library is free software. Distribution and use rights are outlined in
+ * the file "COPYING" which should have been included with this file.  If this
+ * file is missing or damaged, see the license at:
+ *
+ *     https://www.fltk.org/COPYING.php
+ *
+ * Please see the following page on how to report bugs and issues:
+ *
+ *     https://www.fltk.org/bugs.php
+ */
+
+/**
+  Define the minimum Windows target version for compilation.
+
+  Usage:
+  \code
+    #define FL_WIN32_TARGET_VERSION 0x0602  // Windows 8
+    #include <FL/win32_target.h>
+    #include <windows.h>
+  \endcode
+
+  This file should be included at the very beginning of the source file.
+  It should \b not be included in header files to avoid unwanted side
+  effects: including a header file that wants a higher target version
+  would silently raise the target version for the current source file.
+  Although this would usually be benign, it shouldn't be done.
+
+  One notable exception is that you may have to #include fl_config.h or
+  config.h before this file to determine the required target version,
+  but this should be rare.
+
+  For specific version numbers please refer to Microsoft documentation at
+  https://learn.microsoft.com/en-us/cpp/porting/modifying-winver-and-win32-winnt
+
+  This is an excerpt as of July 2026:
+  \code
+    //
+    // _WIN32_WINNT version constants
+    //
+    #define _WIN32_WINNT_NT4          0x0400 // Windows NT 4.0
+    #define _WIN32_WINNT_WIN2K        0x0500 // Windows 2000
+    #define _WIN32_WINNT_WINXP        0x0501 // Windows XP
+    #define _WIN32_WINNT_WS03         0x0502 // Windows Server 2003
+    #define _WIN32_WINNT_WIN6         0x0600 // Windows Vista
+    #define _WIN32_WINNT_VISTA        0x0600 // Windows Vista
+    #define _WIN32_WINNT_WS08         0x0600 // Windows Server 2008
+    #define _WIN32_WINNT_LONGHORN     0x0600 // Windows Vista
+    #define _WIN32_WINNT_WIN7         0x0601 // Windows 7
+    #define _WIN32_WINNT_WIN8         0x0602 // Windows 8
+    #define _WIN32_WINNT_WINBLUE      0x0603 // Windows 8.1
+    #define _WIN32_WINNT_WIN10        0x0A00 // Windows 10
+  \endcode
+*/
+
+#ifndef FL_WIN32_TARGET_VERSION
+#  error "Please define FL_WIN32_TARGET_VERSION before including FL/win32_target.h"
+#endif
+
+/* _WIN32_WINNT */
+#ifndef _WIN32_WINNT
+#  define _WIN32_WINNT FL_WIN32_TARGET_VERSION
+#elif _WIN32_WINNT < FL_WIN32_TARGET_VERSION
+#  undef _WIN32_WINNT
+#  define _WIN32_WINNT FL_WIN32_TARGET_VERSION
+#endif
+
+/* WINVER */
+#ifndef WINVER
+#  define WINVER FL_WIN32_TARGET_VERSION
+#elif WINVER < FL_WIN32_TARGET_VERSION
+#  undef WINVER
+#  define WINVER FL_WIN32_TARGET_VERSION
+#endif
+
+// Note: we can't undefine the macro because this would break the build.
+// #undef FL_WIN32_TARGET_VERSION

src/Fl_win32.cxx

@@ -21,19 +21,10 @@
 // in.  Search other files for "_WIN32" or filenames ending in _win32.cxx
 // for other system-specific code.
 
-/* We require Windows 2000 features (e.g. VK definitions) */
-# if !defined(WINVER) || (WINVER < 0x0500)
-#  ifdef WINVER
-#   undef WINVER
-#  endif
-#  define WINVER 0x0500
-# endif
-# if !defined(_WIN32_WINNT) || (_WIN32_WINNT < 0x0500)
-#  ifdef _WIN32_WINNT
-#   undef _WIN32_WINNT
-#  endif
-#  define _WIN32_WINNT 0x0500
-# endif
+/* We require at least Windows 2000 features (e.g. VK definitions) */
+
+#define FL_WIN32_TARGET_VERSION 0x0500
+#include <FL/win32_target.h>
 
 // recent versions of MinGW warn: "Please include winsock2.h before windows.h"
 #if !defined(__CYGWIN__)
@@ -59,7 +50,9 @@ void fl_cleanup_dc_list(void);
 #include "Fl_Timeout.h"
 #include "print_button.h"
 #include <FL/Fl_Graphics_Driver.H> // for fl_graphics_driver
+#if FLTK_HAVE_PEN_SUPPORT
 #include "drivers/WinAPI/Fl_WinAPI_Pen_Driver.H"
+#endif
 #include "drivers/WinAPI/Fl_WinAPI_Window_Driver.H"
 #include "drivers/WinAPI/Fl_WinAPI_System_Driver.H"
 #include "drivers/WinAPI/Fl_WinAPI_Screen_Driver.H"
@@ -1151,7 +1144,7 @@ static const struct {
   {VK_DIVIDE,   FL_KP+'/'},
   {VK_NUMLOCK,  FL_Num_Lock},
   {VK_SCROLL,   FL_Scroll_Lock},
-#if defined(_WIN32_WINNT) && (_WIN32_WINNT >= 0x0500)
+
   {VK_BROWSER_BACK,     FL_Back},
   {VK_BROWSER_FORWARD,  FL_Forward},
   {VK_BROWSER_REFRESH,  FL_Refresh},
@@ -1167,7 +1160,7 @@ static const struct {
   {VK_MEDIA_STOP,       FL_Media_Stop},
   {VK_MEDIA_PLAY_PAUSE, FL_Media_Play},
   {VK_LAUNCH_MAIL,      FL_Mail},
-#endif
+
   {0xba,        ';'},
   {0xbb,        '='},   // 0xbb == VK_OEM_PLUS (see #1086)
   {0xbc,        ','},

src/drivers/GDI/Fl_GDI_Graphics_Driver_font.cxx

@@ -19,19 +19,11 @@
 #ifndef WIN32_LEAN_AND_MEAN
 # define WIN32_LEAN_AND_MEAN
 #endif
+
 /* We require Windows 2000 features such as GetGlyphIndices */
-#if !defined(WINVER) || (WINVER < 0x0500)
-# ifdef WINVER
-#  undef WINVER
-# endif
-# define WINVER 0x0500
-#endif
-#if !defined(_WIN32_WINNT) || (_WIN32_WINNT < 0x0500)
-# ifdef _WIN32_WINNT
-#  undef _WIN32_WINNT
-# endif
-# define _WIN32_WINNT 0x0500
-#endif
+#define FL_WIN32_TARGET_VERSION 0x0500
+#include <FL/win32_target.h>
+#include <windows.h>
 
 // Select fonts from the FLTK font table.
 #include "Fl_GDI_Graphics_Driver.H"

src/drivers/WinAPI/Fl_WinAPI_Pen_Driver.H

@@ -22,20 +22,6 @@
 
 #include "src/drivers/Base/Fl_Base_Pen_Driver.H"
 
-// The Pointer Input API (GetPointerType, GetPointerPenInfo, POINTER_PEN_INFO,
-// PEN_MASK_*, IS_POINTER_*_WPARAM, ...) requires Windows 8 (_WIN32_WINNT >=
-// 0x0602). Define this *before* <windows.h> is first included anywhere in the
-// translation unit. If FL/Fl.H or another FLTK header pulls in <windows.h>
-// first with a lower target, raise the project-wide _WIN32_WINNT instead
-// (e.g. in fl_config.h) and remove the guard below.
-#ifndef _WIN32_WINNT
-#  define _WIN32_WINNT 0x0602
-#endif
-#ifndef WINVER
-#  define WINVER _WIN32_WINNT
-#endif
-#include <windows.h>
-
 namespace Fl {
 
 namespace Pen {

src/drivers/WinAPI/Fl_WinAPI_Pen_Driver.cxx

@@ -14,6 +14,16 @@
 //     https://www.fltk.org/bugs.php
 //
 
+// The Pointer Input API (GetPointerType, GetPointerPenInfo, POINTER_PEN_INFO,
+// PEN_MASK_*, IS_POINTER_*_WPARAM, ...) requires Windows 8 features
+// (_WIN32_WINNT >= 0x0602). Define this *before* <windows.h> is first included
+// anywhere in the translation unit. See FL/win32_target.h on how to do this.
+
+#define FL_WIN32_TARGET_VERSION 0x0602
+#include <FL/win32_target.h>
+
+#include <windows.h>
+
 /*
  Design notes — Windows Pointer Input vs. Wayland/Cocoa tablet architecture
  ───────────────────────────────────────────────────────────────────────────

test/CMakeLists.txt

@@ -306,7 +306,7 @@ if(CMAKE_CXX_COMPILER_ID IN_LIST _compilers)
   foreach(hd ${all_headers})
     get_filename_component(hd "${hd}" NAME)
     # skip headers that must not be included directly and fl_config.h if it exists
-    string(REGEX MATCH "^(mac|win32|x11|wayland|fl_config|gl[a-z_0-9]*)\.[hH]$" skip "${hd}")
+    string(REGEX MATCH "^(mac|win32.*|x11|wayland|fl_config|gl[a-z_0-9]*)\.[hH]$" skip "${hd}")
     if(skip STREQUAL "")
       file(APPEND ${include_all} "#include <FL/${hd}>\n")
     endif()