configure: Enable clang for older OSX gcc versions

OSX often has a more recent version of clang than it does for GCC. When an older
version of gcc is detected on OSX, the --enable-clang flag is implicitly
enabled.

Author: alexcrichton

configure

@@ -579,26 +579,32 @@ then
         CFG_ENABLE_CLANG=1
         putvar CFG_ENABLE_CLANG
     else
-        # on OS X, with xcode 5 and newer, certain developers may have
-        # cc, gcc and g++ point to a  mixture of clang and gcc
-        # if so, this will create very strange build errors
-        # this last stanza is to detect some such problems and save the future rust
-        # contributor some time solving that issue.
-        # this detection could be generalized to other OSes aside from OS X
-        # but the issue seems most likely to happen on OS X
-
-        chk_cc () {
-            $1 --version 2> /dev/null | grep -q $2
-        }
-        # check that gcc, cc and g++ all point to the same compiler.
-        # note that for xcode 5, g++ points to clang, not clang++
-        if !((chk_cc gcc clang  && chk_cc g++ clang) ||
-            (chk_cc gcc gcc  &&( chk_cc g++ g++ || chk g++ gcc))) then
-            err "the gcc and g++ in your path point to different compilers.
-Check which versions are in your path with  cc --version and g++ --version.
-To resolve this problem, either fix your PATH  or run configure with --enable-clang"
-        fi
+        if [ $("$CFG_GCC" --version 2>&1 | grep -c ' 4\.[0-6]') -ne 0 ]; then
+            step_msg "older GCC found, using clang instead"
+            CFG_ENABLE_CLANG=1
+            putvar CFG_ENABLE_CLANG
+        else
+            # on OS X, with xcode 5 and newer, certain developers may have
+            # cc, gcc and g++ point to a  mixture of clang and gcc
+            # if so, this will create very strange build errors
+            # this last stanza is to detect some such problems and save the future rust
+            # contributor some time solving that issue.
+            # this detection could be generalized to other OSes aside from OS X
+            # but the issue seems most likely to happen on OS X
+
+            chk_cc () {
+                $1 --version 2> /dev/null | grep -q $2
+            }
+            # check that gcc, cc and g++ all point to the same compiler.
+            # note that for xcode 5, g++ points to clang, not clang++
+            if !((chk_cc gcc clang  && chk_cc g++ clang) ||
+                (chk_cc gcc gcc  &&( chk_cc g++ g++ || chk g++ gcc))) then
+                err "the gcc and g++ in your path point to different compilers.
+    Check which versions are in your path with  cc --version and g++ --version.
+    To resolve this problem, either fix your PATH  or run configure with --enable-clang"
+            fi
 
+        fi
     fi
 fi