Clarify ENV.gcc, ENV.llvm c++ compiler names

Library/Homebrew/superenv.rb

@@ -254,11 +254,11 @@ class << ENV
   alias_method :j1, :deparallelize
   def gcc
     ENV['CC'] = ENV['OBJC'] = ENV['HOMEBREW_CC'] = "gcc"
-    ENV['CXX'] = ENV['OBJCXX'] = "g++"
+    ENV['CXX'] = ENV['OBJCXX'] = "g++-4.2"
   end
   def llvm
     ENV['CC'] = ENV['OBJC'] = ENV['HOMEBREW_CC'] = "llvm-gcc"
-    ENV['CXX'] = ENV['OBJCXX'] = "g++"
+    ENV['CXX'] = ENV['OBJCXX'] = "llvm-g++-4.2"
   end
   def clang
     ENV['CC'] = ENV['OBJC'] = ENV['HOMEBREW_CC'] = "clang"