build: @ENABLE_THREADS@ => @LLVM_ENABLE_THREADS@

Makefile.config does not have the value for ENABLE_THREADS substituted as the
variable is called LLVM_ENABLE_THREADS within configure.ac.  This was pointed
out by zygoloid over IRC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174021 91177308-0d34-0410-b5e6-96231b3b80d8