devel/py-cffi: Use OPSYS with OSVERSION, USES+= localbase

r419747 introduced OSVERSION with a ">" comparison without using coupling
with OPSYS, fix.

It also removed CFLAGS and LDFLAGS as "unnecessary", but the fact is they
are still required for non-standard localbases and compilers that don't
search /usr/local/include by default. Restore these flags with the
equivalent USES+=localbase

Approved by: OPVERSION/OPSYS blanket, just-fix-it