Some CMake checks were recently switched from UNIX to LINUX to get them
to not apply to macOS/OSX. Since LINUX doesn't seem to be defined,
switch these checks to UNIX AND NOT APPLE.
Just in case glSwapIntervalEXT and glSwapIntervalSGI aren't available
for whatever reason. This entire patch is most likely completely
redundant on modern mesa drivers.
Ryan Foster
Colin Edwards
jp9000
fryshorts
martell
BtbN