Stop building clang

Removing it from the tree is an ordeal and there is no official way
to disable clang via LLVM's Makefiles so this edits the Makefile
in llvm/tools after running configure.

Author: brson

configure

@@ -881,6 +881,10 @@ do
                 ;;
         esac
         need_ok "LLVM configure failed"
+
+        # Hack the tools Makefile to turn off the clang build
+        sed -i 's/clang//g' tools/Makefile
+
         cd $CFG_BUILD_DIR
     fi