CMake: don't install the internal config.h header

Background:
  http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-June/073707.html

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210766 91177308-0d34-0410-b5e6-96231b3b80d8