Rollup merge of rust-lang#35594 - shepmaster:configure-llvm-39, r=alexcrichton

Allow compiling against a custom LLVM 3.9 installation

Author: eddyb

configure

@@ -1013,7 +1013,7 @@ then
     LLVM_VERSION=$($LLVM_CONFIG --version)
 
     case $LLVM_VERSION in
-        (3.[7-8]*)
+        (3.[7-9]*)
             msg "found ok version of LLVM: $LLVM_VERSION"
             ;;
         (*)