How can I make CMake use GCC instead of Clang on Mac OS X?

CMake doesn’t (always) listen to CC and CXX. Instead use CMAKE_C_COMPILER and CMAKE_CXX_COMPILER:

cmake -DCMAKE_C_COMPILER=/usr/bin/gcc -DCMAKE_CXX_COMPILER=/usr/bin/g++ ...

See also the documentation.

Alternatively, you can provide a toolchain file, but that might be overkill in this case.