Issue #6419: Better clang version detection for Apple Clang

Apple Clang uses different version numbering than "regular" clang, but
it also provides the "regular" version it's based on. Update the sed
pattern to pull out this "regular" version number instead of the Apple
version number.

Author: lilyball

configure

@@ -555,11 +555,11 @@ then
     CFG_CLANG_VERSION=$("$CFG_CLANG" \
                       --version \
                       | grep version \
-                      | sed 's/.*\(version .*\)/\1/' \
+                      | sed 's/.*\(version .*\)/\1/; s/.*based on \(LLVM .*\))/\1/' \
                       | cut -d ' ' -f 2)
 
     case $CFG_CLANG_VERSION in
-        (3.0svn | 3.0 | 3.1* | 3.2* | 3.3* | 4.0* | 4.1* | 4.2*)
+        (3.0svn | 3.0 | 3.1* | 3.2* | 3.3*)
         step_msg "found ok version of CLANG: $CFG_CLANG_VERSION"
         CFG_C_COMPILER="clang"
         ;;