Don't try and use GCC on Xcode 4.2: it's gone Jim.

Closes Homebrew/homebrew#9622.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/extend/ENV.rb

@@ -103,7 +103,7 @@ module HomebrewEnvExtension
     @compiler = :llvm
   end
 
-  def clang
+  def clang args = {}
     self['CC']  = "/usr/bin/clang"
     self['CXX'] = "/usr/bin/clang++"
     replace_in_cflags(/-Xarch_i386 (-march=\S*)/, '\1')

Library/Homebrew/formula.rb

@@ -339,8 +339,13 @@ class Formula
       # latest version we have tested against so we will switch to GCC and
       # bump this integer when Xcode 4.3 is released. TODO do that!
       if llvm.build.to_i >= 2336
-        opoo "Formula will not build with LLVM, using GCC"
-        ENV.gcc :force => true
+        if MacOS.xcode_version < "4.2"
+          opoo "Formula will not build with LLVM, using GCC"
+          ENV.gcc :force => true
+        else
+          opoo "Formula will not build with LLVM, trying Clang"
+          ENV.clang :force => true
+        end
         return
       end
       opoo "Building with LLVM, but this formula is reported to not work with LLVM:"