takes care of the --with-llvmsrc and --with-llvmobj options for the project
It was moved here from the project's configure.ac file because there is
some tricky handling of the llvm_src variable to tell the project where the
llvm source tree is (for mkinstalldirs and install-sh commands).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@20303 91177308-0d34-0410-b5e6-96231b3b80d8
