Always add -fPIC to Fortran compiler flags

Fixes https://github.com/JuliaLang/openlibm/issues/31. Needed in particular to build distribution packages which set their own FFLAGS.

I don't understand why the current ``FFLAGS+=-fPIC`` had no effect, but since the file already does this with CFLAGS, there must be a reason. The result works.