... 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
