GNUMakefile: Explicitly ask for stdlib=libc++ because clang downgrades the version...

GNUMakefile: Explicitly ask for stdlib=libc++ because clang downgrades the version silently if you use backward compatibility mode with OSX 10.5 and then shrink_to_fit() doesn't work.
Config.macosx: Support for shrink_to_fit() is claimed to be in 10.7 and later, but 10.7 and 10.8 both throw this huge error:
https://paste.factorcode.org/paste?id=3872

Fixes #1624.