git-gui: Tighten internal pattern match for lib/ directory
Our GITGUI_LIBDIR macro was testing only for @@ at the start of the path, assuming nobody would ever find that to be a reasonable prefix for a directory to install our library into. That is most likely a valid assumption, but its even more unlikely they would have the start be @@GITGUI_ and the end be @@. Note that we cannot use the full string here because that would get expanded by the sed replacement in our Makefile.
Signed-off-by: Shawn O. Pearce <email@example.com>