Modify the compiler detection test so as to accept "Apple LLVM version
5.1" (etc) and identify it as a Clang variant.  Without that, it gets
misidentified as a gcc variant, which causes problems with Makefile.am's
that use the derived COMPILER_IS_CLANG conditional.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14624 a5019735-40e9-0310-863c-91ae7b9d1cf9