libvpx

Don't try to use getenv on windows phone/rt

The getenv function doesn't exist there. In Visual Studio 2012,
the function still existed in the link libraries even though
it was hidden in the headers, but in the 2013 version it has been
removed from the link libraries as well.

Change-Id: Iea6289a698fa1788e906f5aabb6fddda3675815b