autoconf: generate clang's private config.h header

The CMake build already generated one. Follows clang r149497.

This brings us one step closer to compiling and configuring clang
separately from LLVM using the autoconf build, too.

(I lack the right version of autoconf et al. to regen, but it
was a simple change, so I just updated configure manually.)

llvm-svn: 149498