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.

Closes #23116.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>