Introduce "Modern CMake" in FLTK

This is a big commit and there are too many changes to list them all.
The main changes are:

- rename all CMake build options to 'FLTK_*'
- export library targets with namespace (prefix) 'fltk::'
- standardize shared library target names with suffix '-shared'
- set public build properties on libraries for consumers
- document library names and aliases in README.CMake.txt
- document changes in "Migrating Code from FLTK 1.3 to 1.4"
- partial backwards compatibility for old user projects

Included but not directly related changes:

- fix Windows (Visual Studio) DLL build
- add CMake function fl_debug_target() to show target properties
- don't build test programs if FLTK is a subproject
- internal: reformat CMake code: remove space before '('

Thanks to Matthias and Manolo for their help, testing, and feeback.

zlib/CMakeLists.txt

@@ -1,7 +1,7 @@
 #
 # ZLIB library CMake configuration for the Fast Light Toolkit (FLTK).
 #
-# Copyright 1998-2023 by Bill Spitzak and others.
+# Copyright 1998-2024 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
@@ -16,58 +16,64 @@
 
 # source files for zlib
 
-set (ZLIB_SRCS
-      adler32.c
-      compress.c
-      crc32.c
-      deflate.c
-      gzclose.c
-      gzlib.c
-      gzread.c
-      gzwrite.c
-      inflate.c
-      infback.c
-      inftrees.c
-      inffast.c
-      trees.c
-      uncompr.c
-      zutil.c
+set(ZLIB_SRCS
+    adler32.c
+    compress.c
+    crc32.c
+    deflate.c
+    gzclose.c
+    gzlib.c
+    gzread.c
+    gzwrite.c
+    inflate.c
+    infback.c
+    inftrees.c
+    inffast.c
+    trees.c
+    uncompr.c
+    zutil.c
 )
 
 #######################################################################
 
 # Suppress some Visual Studio compiler warnings
 
-set (msvc_warnings /wd4267 /wd4996)
+set(msvc_warnings /wd4267 /wd4996)
 
 #######################################################################
 # Build the static library
 #######################################################################
 
-FL_ADD_LIBRARY (fltk_z STATIC "${ZLIB_SRCS}")
+fl_add_library(fltk_z STATIC "${ZLIB_SRCS}")
 
-if (MSVC)
-  target_compile_options (fltk_z PRIVATE ${msvc_warnings})
-endif (MSVC)
+if(MSVC)
+  target_compile_options(fltk_z PRIVATE ${msvc_warnings})
+endif(MSVC)
 
 #######################################################################
 # Build the shared library (optional)
 #######################################################################
 
-if (OPTION_BUILD_SHARED_LIBS)
+if(FLTK_BUILD_SHARED_LIBS)
 
-  FL_ADD_LIBRARY (fltk_z SHARED "${ZLIB_SRCS}")
+  # ensure to export all symbols for Windows DLL's
+  set(CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS TRUE)
 
-  if (MSVC)
-    target_compile_options (fltk_z_SHARED PRIVATE ${msvc_warnings})
-  endif (MSVC)
+  fl_add_library(fltk_z SHARED "${ZLIB_SRCS}")
 
-endif (OPTION_BUILD_SHARED_LIBS)
+  if(MSVC)
+    target_compile_options(fltk_z-shared PRIVATE ${msvc_warnings})
+  endif(MSVC)
+
+endif(FLTK_BUILD_SHARED_LIBS)
+
+set(FLTK_LIBRARIES ${FLTK_LIBRARIES} PARENT_SCOPE)
+set(FLTK_LIBRARIES_SHARED ${FLTK_LIBRARIES_SHARED} PARENT_SCOPE)
 
 #######################################################################
 # Install the library headers
 #######################################################################
 
-install (FILES zconf.h zlib.h zutil.h
-         DESTINATION ${FLTK_INCLUDEDIR}/FL/images
+install(FILES zconf.h zlib.h zutil.h
+        DESTINATION ${FLTK_INCLUDEDIR}/FL/images
 )