git-gui: Avoid use of libdir in Makefile

git-gui: Avoid use of libdir in Makefile

Dmitry V. Levin pointed out that on GNU linux libdir is often used
in Makefiles to mean "/usr/lib" or "/usr/lib64", a directory that
is meant to hold platform-specific binary files.  Using a different
libdir meaning here in git-gui's Makefile breaks idomatic expressions
like rpm specifile "make libdir=%_libdir".

Originally I asked that the git.git Makefile undefine libdir before
it calls git-gui's own Makefile but it turns out this is very hard
to do, if not impossible.  Renaming our libdir to gg_libdir resolves
this case with a minimum amount of fuss on our part.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>