xcode_version works for Xcode 4.3 without CLITools

Also correct heinous error when I put `case nil` in the code.

Also outputs "dunno" if we really can't figure it out. I think this is safe.

Library/Homebrew/utils.rb

@@ -378,6 +378,14 @@ module MacOS extend self
 
   def xcode_version
     @xcode_version ||= begin
+      # this shortcut makes xcode_version work for people who don't realise you
+      # need to install the CLI tools
+      xcode43build = "/Applications/Xcode.app/Contents/Developer/usr/bin/xcodebuild"
+      if File.file? xcode43build
+        `#{xcode43build} -version 2>/dev/null` =~ /Xcode (\d(\.\d)*)/
+        return $1 if $1
+      end
+
       # Xcode 4.3 xc* tools hang indefinately if xcode-select path is set thus
       raise if `xcode-select -print-path 2>/dev/null`.chomp == "/"
 
@@ -389,8 +397,8 @@ module MacOS extend self
       # 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"
+      case llvm_build_version.to_i
+      when 1..2063 then "3.1.0"
       when 2064..2065 then "3.1.4"
       when 2366..2325
         # we have no data for this range so we are guessing
@@ -405,7 +413,9 @@ module MacOS extend self
         "4.0"
       else
         case (clang_version.to_f * 10).to_i
-        when 0..14
+        when 0
+          "dunno"
+        when 1..14
           "3.2.2"
         when 15
           "3.2.4"