Summary:
This makes sure that one can run `./build-infer.sh` then `make`. Otherwise it's
not always clear what one should do to recompile infer, eg when `make` will
work and when `./build-infer.sh` should be used instead, in particular when the
user doesn't have opam configured for her terminal.
Reviewed By: jberdine
Differential Revision: D4698159
fbshipit-source-id: 5df8059