CMake: Fail if src/config.h exists

This will break the build since the autotools config.h and cmake's
config.h may differ slightly.

Change-Id: Ifd7f3cd23192120e74865c02dd604a03f3fe6a94
Reviewed-on: http://review.couchbase.org/43090
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>