Touch configure to force clang's config.h.in reconfiguration on the build servers

Gutted that this is needed, folks who care about keeping the legacy build
system alive should sort this out already.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210321 91177308-0d34-0410-b5e6-96231b3b80d8