Log Message:
x11/motif: Work around quirks in build-time tool-building.

See comments for details.  Tidy up variable definitions while here.
Use native ar/ranlib if available -- armv7--netbsdelf-eabihf-ranlib
doesn't seem to do anything on an archive of x86 objects to build a
tool library, so let's try using the native one instead for such a
cross-build.