This also adds gmp and mpfr as optional dependencies, and sets the
sysroot to to allow gawk to find libgmp when building with Clang, and
to make the results predictable regardless of the order the ports are
installed in.
All of these patches did the same thing, which is already in upstream
config.sub.
With this change, we need only add `use_fresh_config_sub=true` to
the package.sh file.
Note that this is not done automatically in case the port has a modified
config.sub file.
We may need entries with spaces in makeopts, installopts, and
configopts, and at that point we should also convert depends and
auth_opts to avoid confusion.
