Modified the makefile to define LIBCLANG_MAJOR, LIBCLANG_MINOR, and
LIBCLANG_PATCHLEVEL based on the llvm-config executable in the llvm
directory pointed to by the configuration. These new variable names
are independent of the __clang_major__, etc. variables we were defining
only in Linux. Using these names, we can reduce the clang version
checks to a single consistent set of version numbers. We don't have
to deal with the different version numbers assigned by Apple.