do not warn about c99 extensions
do not warn about c99 extensions

Test Plan:
use a c99 feature in the code that used to cause a warning and see that no
warning is emitted with this diff.

Reviewers: akotulski, mathieubaudet

Reviewed By: mathieubaudet

Subscribers: devonharris

Differential Revision: https://phabricator.fb.com/D2300298

Signature: t1:2300298:1438354126:7daf2a65c23cc91ae699dcd270fc504bcf6472ec

CJstar/facebook-clang-plugins,raphaelamorim/facebook-clang-plugins,CJstar/facebook-clang-plugins,raphaelamorim/facebook-clang-plugins,CJstar/facebook-clang-plugins,raphaelamorim/facebook-clang-plugins,raphaelamorim/facebook-clang-plugins,CJstar/facebook-clang-plugins