Add ENV.cppflags shortcut

Library/Homebrew/extend/ENV.rb

@@ -179,6 +179,7 @@ module HomebrewEnvExtension
   def cc;      self['CC'] or "gcc";  end
   def cxx;     self['CXX'] or "g++"; end
   def cflags;  self['CFLAGS'];       end
+  def cppflags;self['CPPLAGS'];      end
   def ldflags; self['LDFLAGS'];      end
 
   def m64