Fail the configure script if you don't have boost

Fail the configure script if you don't have boost

Summary:
Discovered this while trying to build folly on OS X. It would print a
warning about Boost being missing that got drowned out in the rest of
the configure output, and then the build would fail. This makes the
failure more friendly.

Test Plan:
`autoreconf; ./configure` on a system without boost
installed; make sure configure fails and the last thing it prints is the
boost error message.

Reviewed By: delong.j@fb.com

FB internal diff: D797389