build: set up basic multi-arch/mode build

Make `make -f Makefile.build ia32.release x64.debug` work.  It's not
perfect yet: it requires running `./configure` first and the generated
`config.gypi` is shared across builds.

PR-URL: https://github.com/node-forward/node/pull/24


Reviewed-By: Fedor Indutny <fedor@indutny.com>