Add ENV.clang; Fixes Homebrew/homebrew#6580

Also tidied it up a little, reducing duplication as per original patch.

Library/Homebrew/extend/ENV.rb

@@ -27,16 +27,11 @@ module HomebrewEnvExtension
 
     if MACOS_VERSION >= 10.6
       if self.use_clang?
-        self['CC']  = "#{MacOS.xcode_prefix}/usr/bin/clang"
+        self.clang
-        self['CXX'] = "#{MacOS.xcode_prefix}/usr/bin/clang++"
+      elsif self.use_llvm?
-      elsif self.use_llvm? and MacOS.xcode_version < '4.1'
+        self.llvm
-        # With Xcode 4 cc is llvm
+      elsif self.use_gcc?
-        self['CC']  = "#{MacOS.xcode_prefix}/usr/bin/llvm-gcc"
+        self.gcc
-        self['CXX'] = "#{MacOS.xcode_prefix}/usr/bin/llvm-g++"
-      elsif self.use_gcc? and MacOS.xcode_version < '4'
-        # With Xcode4 cc, c++, gcc and g++ are actually symlinks to llvm-gcc
-        self['CC']  = "#{MacOS.xcode_prefix}/usr/bin/gcc-4.2"
-        self['CXX'] = "#{MacOS.xcode_prefix}/usr/bin/g++-4.2"
       end
     end
 
@@ -132,19 +127,32 @@ module HomebrewEnvExtension
   end
   alias_method :gcc_4_0, :gcc_4_0_1
 
-  def gcc_4_2
+  def gcc
-    # Sometimes you want to downgrade from LLVM to GCC 4.2
+    if MacOS.xcode_version < '4'
-    self['CC']="/usr/bin/gcc-4.2"
+      self['CC'] = '/usr/bin/cc'
-    self['CXX']="/usr/bin/g++-4.2"
+      self['CXX'] = '/usr/bin/c++'
-    self['LD']=self['CC']
+    else
-    self.O3
+      # With Xcode4 cc, c++, gcc and g++ are actually symlinks to llvm-gcc
+      self['CC']  = "#{MacOS.xcode_prefix}/usr/bin/gcc-4.2"
+      self['CXX'] = "#{MacOS.xcode_prefix}/usr/bin/g++-4.2"
+    end
+    remove_from_cflags '-O4'
   end
+  alias_method :gcc_4_2, :gcc
 
   def llvm
-    self['CC'] = "#{MacOS.xcode_prefix}/usr/bin/llvm-gcc"
+    if MacOS.xcode_version < '4.1'
-    self['CXX'] = "#{MacOS.xcode_prefix}/usr/bin/llvm-g++"
+      self['CC'] = "#{MacOS.xcode_prefix}/usr/bin/llvm-gcc"
-    self['LD'] = self['CC']
+      self['CXX'] = "#{MacOS.xcode_prefix}/usr/bin/llvm-g++"
-    self.O4
+    else
+      self['CC'] = '/usr/bin/cc'
+      self['CXX'] = '/usr/bin/c++'
+    end
+  end
+
+  def clang
+    self['CC'] = "#{MacOS.xcode_prefix}/usr/bin/clang"
+    self['CXX'] = "#{MacOS.xcode_prefix}/usr/bin/clang++"
   end
 
   def fortran