to parse the GMP ./configure output and generate part of a makefile.
move all the mpn build stuff into the MD part of the framework, and
update the amd64 port to build all the parts it should.
XXX: amd64 build fails to preprocess redc_1.asm gcd_1.asm mod_1_4.asm,
XXX: so we're using the generic C versions for now.