Make xcode_version work with CLI4.3X

Closes Homebrew/homebrew#10460.

Library/Homebrew/utils.rb

@@ -363,10 +363,10 @@ module MacOS extend self
       raise if $1.nil? or not $?.success?
       $1
     rescue
-      # for people who don't have xcodebuild installed due to using
-      # some variety of minimal installer, let's try and guess their
-      # Xcode version
-      case llvm_build_version.to_i
+      # For people who's xcode-select is unset, or who have installed
+      # xcode-gcc-installer or whatever other combinations we can try and
+      # supprt. See https://github.com/mxcl/homebrew/wiki/Xcode
+      case nil
       when 0..2063 then "3.1.0"
       when 2064..2065 then "3.1.4"
       when 2366..2325
@@ -381,7 +381,24 @@ module MacOS extend self
         # https://github.com/mxcl/homebrew/wiki/Xcode
         "4.0"
       else
-        "4.2"
+        case (clang_version.to_f * 10).to_i
+        when 0..14
+          "3.2.2"
+        when 15
+          "3.2.4"
+        when 16
+          "3.2.5"
+        when 17..20
+          "4.0"
+        when 21
+          "4.1"
+        when 22..30
+          "4.2"
+        when 31
+          "4.3"
+        else
+          "4.3"
+        end
       end
     end
   end