and, to continue in the same logic, I would also suggest to create a new file

FL/x11.H and to put there the X11-specific part of FL/platform.H which would

then contain, in essence :

#   include <FL/platform_types.h> // will bring in FL/fl_config.h

#  ifdef _WIN32
#    include "win32.H"
#  elif defined(__APPLE__)
#    include "mac.H"
#  elif defined(__ANDROID__)
#    include "android.H"
#  elif defined(FLTK_USE_X11)

#     include "x11.H"

#  endif

//
// cross-platform declarations
//

……