Revert "jni: build with gcc for now"

This reverts commit 9b45ac1955db49cddb4dbe28619b87eb24915cd3.
jni: fix c99 inline linking

Summary:
Clang builds C code according to the C99 standard,
which provides different semantics for the inline
keyword than GCC's default behavior. This is the
correct behavior without any gnu extensions.

See here:
http://clang.llvm.org/compatibility.html#inline

Test Plan: Nearly verified

Reviewers: randall.rushing

Differential Revision: http://review.codefirex.com/D662