Enable AVX opt for Win builds

Target vs12 == Visual Studio 2013 during configure.
This enables HAVE_AVX[2]

Add MSVS flags for /arch:AVX via
EnableEnhancedInstructionSet: 3
Conveniently added in recently:
https://codereview.chromium.org/195283002

R=tomfinegan@chromium.org
TBR=tomfinegan@chromium.org

Review URL: https://codereview.chromium.org/239003004

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/libvpx@264025 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
9 files changed