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Hard-code CC and CXX to clang for Macs, since gcc/g++ will *not* work with
Anaconda, despite the fact that distutils will try to use them.  System Python,
homebrew, and MacPorts on Macs will always use clang, so this hack should always
work and fix issues with users that have GCC installed from MacPorts or homebrew
*and* Anaconda.