autoconf: add private config.h to clang

autoconf: add private config.h to clang

This already exists in the CMake build, which is part of what makes
building clang separately from llvm via cmake possible. This cleans up
that discrepancy between the build systems (and sets the groundwork
for configuring clang separately, too).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@149497 91177308-0d34-0410-b5e6-96231b3b80d8