Add ENV.cxxflags

Useful when CXXFLAGS has diverged fom CFLAGS, e.g. via ENV.append, and
also it is nice for CXX to have symmetry with CC.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/extend/ENV.rb

@@ -211,6 +211,7 @@ Please take one of the following actions:
   def cc;      self['CC'] or "gcc";  end
   def cxx;     self['CXX'] or "g++"; end
   def cflags;  self['CFLAGS'];       end
+  def cxxflags;self['CXXFLAGS'];     end
   def cppflags;self['CPPLAGS'];      end
   def ldflags; self['LDFLAGS'];      end