Refactor compiler wrapper

Since setting *FLAGS seems to have had unforeseen (by me, anyway) consequences, this branch uses the compiler wrapper instead to add -Wno-error=unused-command-line-argument-hard-error-in-future to each clang invocation.  In addition to adding the flag:

1) I added the path-prefix-clang directory for 10.9, so that its 'clang', 'cc' and 'gcc' would benefit from the wrapper.
2) path-prefix-clang/ and path-prefix-libcxx are created dynamically the first time a package is built, rather than being stored in the .deb.  Unfortunately, Fink::PkgVersion::ensure_libcxx_prefix() and Fink::PkgVersion::ensure_clang_prefix() don't allow for updates to be made if the respective compiler_wrapper files exist.  The simplest option seemed to be to remove the directories in postinstall, and create updates dynamically during the next build operation.  We could also create them in the .deb as well, with a bit more work.