Tell the user to use CMAKE_TOOLCHAIN_FILE upon failure.
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Cleanup detectOperatingSystem. Should be the same but adding
GNU and kFreeBSD from Debian and filtering out pre OS X Apple.
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libaio is linux only. Also check for the correct header since
aio.h is the POSIX one. Both are in /usr/include/ anyway.
Only build core if (Linux AND NOT AIO_FOUND) == false.
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Use Unix for GTK and X11. Macs has gtk-quartzs and xquartz or native X11.
*BSD, linux, etc should have both.