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.
Linking was failing due to inline function handling changes between gcc (ISOGNU89) and clang (ISOGNU99).
In our currently used gcc utilizing ISOGNU11 by default, the inline function differences no longer apply.

Test Plan: test and win

Reviewers: randall.rushing, synergy

Reviewed By: synergy

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