provides msvc-like warnings for implicit conversions from 64-bit to
32-bit types

--enable-vp9-highbitdepth still requires some work

this also skips CXXFLAGS for now as some work would be needed to cleanup
third_party/*.cc or split it from test/*.cc where it comes to flags.

Change-Id: Ic9a095b73286eba5ed39bfc27ff69593748cbbf4