Fix GMP_BUILD_DIR in Makefile.include.
Remove redundant variable decls in gmp Makefile.
Update gmp Makefile to use real files as targets, for clarity and consistency.

Replace the 'gmp-config' and 'gmp-build' targets with files that are generated
by those recipes (for clarity). Also, pull the build/ subdir creation out of
'gmp-config' into a separate target that runs first. This will be important for
the gmp speculative build.
Only try to speculatively build gmp once, even if it fails.

Use the gmp build/ subdir, which is created immediately, as an indication that
the gmp build has been attempted. If it exists, do not retry.