Log Message:
devel/gps: Disable on NetBSD 5 and 6, serious link issues

I've spent upwards on a day trying to fix a massive link failure on
GPS that appeared with the new gcc-aux compiler.  My theory is that
the binutils 2.21 used with NetBSD 6 is not new enough for gcc 4.9.0.
However, gcc-aux will not build with devel/binutils -- there seems to
be a problem with handing PIC, at least on NetBSD.

For now, disable GPS building on NetBSD releases.  I will try again
with NetBSD 6.99 which uses binutils 2.23.  If that succeeds, then I'll
submit a PR against devel/binutils.  I've tried everything I can think
of to get GPS to build on x86-64-NetBSD-6.1.4 but everything has failed.