Said executables will not be used for anything, of course.

We can't use gb_STDLIBS, which also contains -lm after my previous
commit, as fontconfig is C, not C++, and gb_STDLIBS contains
-lgnustl_static, and only our $(CXX), not $(CC), contains the -L that
points to where -lgnustl_static is to be found.

Change-Id: I40c459580f357d913ddc55eae00e16f90f81d510