Also pulled Windows-specific modifications to config.h.in to a separate file, so
that changes to it trigger regeneration of config.h (which doesn't happen when the
custom build step for it changes).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@25477 91177308-0d34-0410-b5e6-96231b3b80d8