libvpx

Fix building for arm with Visual Studio 2013

The microsoft build tools explicitly disallow building for arm in
the "desktop" target configuration; one has to target "Windows
Store" apps (aka WinRT/Metro) or Windows Phone. In Visual Studio
2012, one could just pick the v110_wp80 toolset which made the
vcxproj files buildable. In Visual Studio 2013, picking the v120_wp81
toolset isn't enough - one has to configure the vcxproj files
as an "AppContainerApplication". This has the implication that
you can't just build a plain .exe (such as the examples) - an .exe
project would need to have an AppxManifest file. Therefore we can
only build the library itself.

If loaded into Visual Studio for Windows (the Windows Store/Phone
version of Visual Studio, not the Desktop one), the obj_int_extract
project is omitted since it's treated as incompatible. Building
from the command line with msbuild works fine though.

The armv7-win32-vs12 target was added as part of a638bdf4 even
though actual use of it hadn't been tested.

Change-Id: Iee8088252cf790317aeb6b417d29058225f1f629