scons: Define _ALLOW_KEYWORD_MACROS on MSVC builds.

scons/llvm.py defines inline globally to workaround issues with LLVM C
binding headers, so the only way to is to avoid
aggravating xkeycheck.h errors is to set _ALLOW_KEYWORD_MACROS.

This fixes MSVC 2012 build with LLVM.

Reviewed-by: Brian Paul <brianp@vmware.com>
2 files changed