Add --cc= syntax for selecting compilers

Library/Homebrew/extend/ARGV.rb

@@ -183,6 +183,10 @@ def filter_for_dependencies
     replace(old_args)
   end
 
+  def cc
+    value 'cc'
+  end
+
   private
 
   def downcased_unique_named

Library/Homebrew/extend/ENV/shared.rb

@@ -2,6 +2,8 @@ module SharedEnvExtension
   CC_FLAG_VARS = %w{CFLAGS CXXFLAGS OBJCFLAGS OBJCXXFLAGS}
   FC_FLAG_VARS = %w{FCFLAGS FFLAGS}
 
+  COMPILERS = ['clang', 'gcc-4.0', 'gcc-4.2', 'llvm-gcc']
+
   def remove_cc_etc
     keys = %w{CC CXX OBJC OBJCXX LD CPP CFLAGS CXXFLAGS OBJCFLAGS OBJCXXFLAGS LDFLAGS CPPFLAGS}
     removed = Hash[*keys.map{ |key| [key, self[key]] }.flatten]

Library/Homebrew/extend/ENV/std.rb

@@ -344,7 +344,9 @@ def compiler
     # test for --flags first so that installs can be overridden on a per
     # install basis. Then test for ENVs in inverse order to flags, this is
     # sensible, trust me
-    @compiler ||= if ARGV.include? '--use-gcc'
+    @compiler ||= if (cc = ARGV.cc)
+      COMPILERS.grep(cc).pop ? cc : raise("Invalid value for --cc: #{cc}")
+    elsif ARGV.include? '--use-gcc'
       :gcc
     elsif ARGV.include? '--use-llvm'
       :llvm

Library/Homebrew/extend/ENV/super.rb

@@ -122,7 +122,9 @@ def m32
   private
 
   def determine_cc
-    if ARGV.include? '--use-gcc'
+    if (cc = ARGV.cc)
+      COMPILERS.grep(cc).pop ? cc : raise("Invalid value for --cc: #{cc}")
+    elsif ARGV.include? '--use-gcc'
       gcc_installed = Formula.factory('apple-gcc42').installed? rescue false
       # fall back to something else on systems without Apple gcc
       if MacOS.locate('gcc-4.2') || gcc_installed