How do I specify what is used as the real C compiler?
Apple's gcc binary is really llvm.

Mac:~ bmike$ which gcc
/usr/bin/gcc
Mac:~ bmike$ gcc --version
i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


You'll want to obtain gcc from homebrew or from source if you need gcc and not llvm standing in for gcc.

Good reading abounds on various blogs:


http://kennethreitz.org/experiments/xcode-gcc-and-homebrew
http://jfire.io/blog/2012/03/02/xcode-4-dot-3-homebrew-and-ruby/
http://www.andrewsavory.com/blog/2012/2458
https://github.com/mxcl/homebrew/wiki/Custom-GCC-and-cross-compilers


Once you've gotten a real version of gcc installed, you can export CC=/usr/local/bin/gcc or whatever else you prefer for setting the environment variable to match were you choose to store your now non-llvm-based compiler.