CMake/macOSMaxAllowed.c:
avoid C compiler warning in test code
CMake/setup.cmake, technical changes:
- optimization: test SDK 15.0.0 first to avoid unnecessary test if true
- use a function instead of a macro
- use different names for (cached) result variables
- use prefix FLTK_ for cache variables
Note: the cache variables are marked 'internal': you may need to grep
CMakeCache.txt to view them.
