Allow :force for ENV.gcc

This is used with fails_with_llvm so that if GCC cannot be found (and we suspect this is the case with Xcode 4.2) the build will fail.

Quite possibly it should be the default.

Library/Homebrew/extend/ENV.rb

@@ -127,11 +127,15 @@ module HomebrewEnvExtension
   end
   alias_method :gcc_4_0, :gcc_4_0_1
 
-  def gcc
+  def gcc args = {}
     self['CC']  = "/usr/bin/gcc-4.2"
     self['CXX'] = "/usr/bin/g++-4.2"
     remove_from_cflags '-O4'
     @compiler = :gcc
+
+    raise "GCC could not be found" if args[:force] and not File.exist? ENV['CC'] \
+                                   or (File.symlink? ENV['CC'] \
+                                   and File.readlink(ENV['CC']) =~ 'llvm')
   end
   alias_method :gcc_4_2, :gcc
 

Library/Homebrew/formula.rb

@@ -327,7 +327,7 @@ class Formula
       # bump this integer when Xcode 4.2 is released. TODO do that!
       if llvm.build.to_i >= 2335
         opoo "Formula will not build with LLVM, using GCC"
-        ENV.gcc
+        ENV.gcc :force => true
         return
       end
       opoo "Building with LLVM, but this formula is reported to not work with LLVM:"