problem: trying to link against librt on macOS

[fanquake]

Solution: don't use rt if not RT_LIBRARY

This currently causes the check to fail on macOS:

/Library/Developer/CommandLineTools/usr/bin/c++  -std=c++11 -Wno-tautological-constant-compare  -arch arm64 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX14.4.sdk -Wl,-search_paths_first -Wl,-headerpad_max_install_names CMakeFiles/cmTC_8466b.dir/CheckSymbolExists.cxx.o -o cmTC_8466b  -lrt
ld: library 'rt' not found
clang: error: linker command failed with exit code 1 (use -v to see invocation)

even though clock_gettime is available.

Move the clock_gettime check up with the other librt related checks, and don't set CMAKE_REQUIRED_LIBRARIES if not RT_LIBRARY.