libpng: do not lower standard to isognu89

Summary: This should no longer be necessary. Clang defaults to 99 which is why this change existed in the first place. runtime linking was failing due to inline function handling changes between gcc and clang on gcc This should no longer be necessary. Clang defaults to 99 which is why this change existed in the first place. runtime linking was failing due to inline function handling changes between gcc and clang on gcc 4.6. In our currently used gcc 4.8, the inline function handling has been fixed.

Test Plan: test and win

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D427