[libvpx] Fix compilation on Mavericks.

libc++ doesn't include the C++ TR1 extensions which libvpx needs, so
force it to use its own built-in tuple code by passing
-DGTEST_USE_OWN_TR1_TUPLE=1 to the compiler.

I didn't put a guard so it only happens on Mavericks, but it shouldn't hurt on other OS versions.