Generate FL/fl_config.h rather than FL/abi-version.h

... as discussed in fltk.coredev in thread "RFC: introduce public
config header <FL/fl_config.h>", see:
https://groups.google.com/g/fltkcoredev/c/xLCs1AIXMVo/m/MHZpQggzAQAJ

- Rename abi-version.h to fl_config.h, rename input files,
  update dependencies, .gitignore, CMake, configure and Makefiles.

- Include Cairo options in FL/fl_config.h

- Rename FLTK_USE_CAIRO to FLTK_HAVE_CAIROEXT for consistency.

- Include <FL/fl_config.h> in config.h and wherever necessary,
  fix include order (move FL/Fl.H to the top) and more.

- Move USE_X11 to fl_config.h and rename to FLTK_USE_X11

- Do not include <config.h> in Cairo demo program which is no
  longer required in Cairo programs since FLTK 1.4.0

FL/Enumerations.H

@@ -24,13 +24,13 @@
 /*
  ******************************************************************************
  * FL_ABI_VERSION is defined by configure or CMake since FLTK 1.3.4.
- * It is written to FL/abi-version.h and #included here.
+ * It is written to FL/fl_config.h and #included here.
  ******************************************************************************
  * For more informations on FL_ABI_VERSION see README.abi-version.txt.
  ******************************************************************************
  */
 
-#include <FL/abi-version.h>
+#include <FL/fl_config.h>
 
 #  include "Fl_Export.H"
 #  include "fl_types.h"

FL/Fl.H

@@ -21,6 +21,7 @@
 #ifndef Fl_H
 #  define Fl_H
 
+#include <FL/fl_config.h> // build configuration
 #include <FL/Fl_Export.H>
 #include <FL/platform_types.h> // for FL_SOCKET
 #include <FL/fl_casts.H>       // experimental

FL/Fl_Cairo.H

@@ -1,7 +1,7 @@
 //
 // Main header file for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2016 by Bill Spitzak and others.
+// Copyright 1998-2021 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
@@ -20,6 +20,9 @@
 
 #ifndef FL_CAIRO_H
 # define FL_CAIRO_H
+
+# include <FL/fl_config.h> // build configuration
+
 # ifdef FLTK_HAVE_CAIRO
 
 // Cairo is currently supported for the following platforms:
@@ -39,7 +42,9 @@
    A private internal & unique corresponding object is created to
    permit cairo context state handling while keeping it opaque.
    For internal use only.
-   \note Only available when configure has the --enable-cairo option
+   \note Only available when configure has the --enable-cairo or
+        --enable-cairoext option or one or both of the CMake options
+        OPTION_CAIRO or OPTION_CAIROEXT is set (ON)
 */
 class FL_EXPORT Fl_Cairo_State {
 public:

FL/Fl_Cairo_Window.H

@@ -1,7 +1,7 @@
 //
-// Main header file for the Fast Light Tool Kit (FLTK).
+// Cairo Window header file for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2018 by Bill Spitzak and others.
+// Copyright 1998-2021 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
@@ -20,6 +20,9 @@
 
 #ifndef FL_CAIRO_WINDOW_H
 # define FL_CAIRO_WINDOW_H
+
+#include <FL/fl_config.h>
+
 # ifdef FLTK_HAVE_CAIRO
 
 // Cairo is currently supported for the following platforms:

FL/platform.H

@@ -1,7 +1,7 @@
 //
 // Platform header file for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2020 by Bill Spitzak and others.
+// Copyright 1998-2021 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
@@ -28,6 +28,7 @@
 #if !defined(FL_PLATFORM_H) && !defined(FL_DOXYGEN)
 #  define FL_PLATFORM_H
 
+#   include <FL/fl_config.h>
 #   include <FL/Fl_Export.H>
 #   include <FL/platform_types.h>
 #   include <FL/fl_types.h> // for uchar
@@ -42,9 +43,6 @@ class Fl_Window;
 #  else // X11
 #   include <FL/fl_types.h>
 #   include <FL/Enumerations.H>
-#    if !defined(USE_X11)
-#      define USE_X11 1
-#    endif
 #    if defined(_ABIN32) || defined(_ABI64) // fix for broken SGI Irix X .h files
 #      pragma set woff 3322
 #    endif
@@ -125,7 +123,7 @@ public:
   // static variables, static functions and member functions
   static Fl_X* first;
   static Fl_X* i(const Fl_Window* w) {return (Fl_X*)w->i;}
-#  if defined(USE_X11) // for backward compatibility
+#  if defined(FLTK_USE_X11) // for backward compatibility
   static void make_xid(Fl_Window*, XVisualInfo* =fl_visual, Colormap=fl_colormap);
   static Fl_X* set_xid(Fl_Window*, Window);
 #  endif