scons: Support LLVM 3.5 and 3.6 on windows.

llvm/Config/llvm-config.h is parsed instead of llvm/Config/config.h for
detecting LLVM version
(http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-June/073707.html).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
1 file changed