ruby-build: add autoconf and pkg-config deps.

When compiling ruby versions with ruby-build (and of course, rbenv)
from a brand new Mountain Lion (10.8.2) setup and no other formulas,
the build process fails by missing autoconf and pkg-config.

Tested with ruby v2.0.0-dev and v1.9.3-p385.

Thanks to Pierre Paridans.

Closes #18016.