valgrind: doesn't build with an old clang

[mojca (Mojca Miklavec)]

valgrind fails to build already at configure time if clang from xcode 425 is used, complaining about too old compiler.

A naive addition to the Portfile that blacklisted /usr/bin/clang++ and llvm-gcc-4.2 made the build succeed:

PortGroup           compiler_blacklist_versions 1.0
compiler.blacklist  *gcc* {clang < 500}

but I didn't test macports-clang-3.3 or xcode 5. I'm also not sure whether the port needs some special handling about gcc (I don't understand what the rest of the portfile does).