Summary:
This will make it easier to bisect on top of this commit, as values in
Makefile.autoconf.in are not expected to change across commits, but
Makefile.config may fluctuate.
This should also make it easier to rebase stacks of diffs without having
to rerun `./autogen.sh && ./configure`.
Reviewed By: jberdine
Differential Revision: D4410077
fbshipit-source-id: 1119fb6