Build libwebrtc using libcxx. Set use_custom_libcxx=0. According to the
source comment, use_custom_libcxx=1 means to use a precompiled libcxx in
the third_party directory.

Build the test target
obj/webrtc/api/test/peerconnection_unittests.fakeaudiocapturemodule.o
rather than peerconnection_unittests.isolated; building just an object
file doesn't require the linker and is faster.

Use x86_64-apple-darwin11-ar, not ar, to link libwebrtc-magic.a.
When linking go-webrtc later on, it would cause the error:
ld: warning: ignoring file lib/libwebrtc-darwin-amd64-magic.a, file was built for archive which is not the architecture being linked (x86_64)