scons: Don't use bundled C99 headers for VS 2013.
Use the ones provided by the compiler instead. NOTE: External trees should be updated to not include '#include/c99' directory directly, but rather rely on scons/gallium.py to do the right thing. Reviewed-by:Brian Paul <brianp@vmware.com> Reviewed-by:
Roland Scheidegger <sroland@vmware.com>