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

fluid/CMakeLists.txt

@@ -107,6 +107,12 @@ else ()
 endif (APPLE AND (NOT OPTION_APPLE_X11) AND (NOT OPTION_APPLE_SDL))
 
 target_link_libraries (fluid fltk fltk_images)
+
+# we must link all programs with fltk_cairo if option CAIROEXT is enabled
+if (FLTK_HAVE_CAIROEXT)
+  target_link_libraries (fluid fltk_cairo cairo)
+endif (FLTK_HAVE_CAIROEXT)
+
 if (FLTK_HAVE_CAIRO)
   fl_target_link_directories (fluid PRIVATE "${PKG_CAIRO_LIBRARY_DIRS}")
 endif (FLTK_HAVE_CAIRO)