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.

1 files changed, 8 insertions, 7 deletions

diff --git a/src/Fl_Cairo.cxx b/src/Fl_Cairo.cxx
index 598c8747e..4116bb1ac 100644
--- a/src/Fl_Cairo.cxx
+++ b/src/Fl_Cairo.cxx
@@ -1,7 +1,7 @@
 //
 // Special Cairo support for the Fast Light Tool Kit (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
@@ -14,13 +14,14 @@
 //     https://www.fltk.org/bugs.php
 //
 
-// This file implements the FLTK Cairo support (since 1.3.x):
+// This file implements the FLTK Cairo Window support (since 1.3.x):
 //
 // - ./configure --enable-cairo  and/or  --enable-cairoext
-// - cmake       -DOPTION_CAIRO  and/or  -DOPTION_CAIROEXT
+// - cmake       -D FLTK_OPTION_CAIRO_WINDOW  and/or  -D FLTK_OPTION_CAIRO_EXT
 //
 // Preprocessor macro FLTK_HAVE_CAIRO is defined for both options.
-// Preprocessor macro FLTK_HAVE_CAIRO_EXT is defined only for "cairoext"
+// Preprocessor macro FLTK_HAVE_CAIRO_EXT is defined only for "cairoext".
+// Both macros are defined in 'FL/fl_config.h'.
 
 #include <FL/Fl.H> // includes <FL/fl_config.h>
 
@@ -69,7 +70,7 @@ void Fl_Cairo_State::autolink(bool b) {
   autolink_ = b;
 #else
   Fl::fatal("In Fl::autolink(bool): Cairo autolink() feature is only "
-            "available with CMake OPTION_CAIROEXT "
+            "available with CMake FLTK_OPTION_CAIRO_EXT "
             "or the enable-cairoext configure option.\n"
             "Quitting now.");
 #endif
@@ -167,7 +168,7 @@ static cairo_surface_t *cairo_create_surface(void *gc, int W, int H) {
   Creates a Cairo context from a \a gc only, gets its window size or
   offscreen size if fl_window is null.
 
-  \note Only available if CMake OPTION_CAIRO is enabled
+  \note Only available if CMake FLTK_OPTION_CAIRO_WINDOW is enabled
         or configure has the --enable-cairo option.
 */
 cairo_t *Fl::cairo_make_current(void *gc) {
@@ -210,7 +211,7 @@ cairo_t *Fl::cairo_make_current(void *gc) {
 /**
   Creates a Cairo context from a \p gc and the given size.
 
-  \note Only available if CMake OPTION_CAIRO is enabled
+  \note Only available if CMake FLTK_OPTION_CAIRO_WINDOW is enabled
         or configure has the --enable-cairo option.
 */
 cairo_t *Fl::cairo_make_current(void *gc, int W, int H) {