Add optional description which complains on newer versions of autoconf

Summary: https://github.com/facebook/folly/pull/22

Test Plan: none

Reviewed By: andrewjcg@fb.com

FB internal diff: D683791
Boost system is now needed

Summary: https://github.com/facebook/folly/pull/22

Test Plan: .

Reviewed By: andrewjcg@fb.com

FB internal diff: D683793