Bug 763495 - Restore exports, needed by webrtc on Android.

I don't know why android is different from desktop, but it doesn't
generate an include line to the srcdir copy.
Bug 763495 - Use gmake instead of pymake.

pymake doesn't understand the libvpx makefile. Fall back to gmake
the same way the b2g gaia build does.

NB: we may need -j1 on windows with msys make as well. The gaia
makefile passes it unconditionally.