auto merge of #4787 : graydon/rust/clang42, r=graydon

This is just a retargeting of #4753 against incoming

Author: bors

configure

@@ -515,7 +515,7 @@ then
                       | cut -d ' ' -f 2)
 
     case $CFG_CLANG_VERSION in
-        (3.0svn | 3.0 | 3.1* | 3.2* | 4.0* | 4.1*)
+        (3.0svn | 3.0 | 3.1* | 3.2* | 4.0* | 4.1* | 4.2*)
         step_msg "found ok version of CLANG: $CFG_CLANG_VERSION"
         CFG_C_COMPILER="clang"
         ;;