Update WITH_/WITHOUT_CLANG_IS_CC descriptions

Describe /usr/bin/cc etc. as links to the compiler, and don't conflate
WITHOUT_CLANG_IS_CC with installing GCC. Leave a reference to WITH_GCC
and WITHOUT_CLANG_IS_CC installing links to GCC, although this will be
removed in ~1.5 months when GCC 4.2.1 is removed from the tree.

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