message: go: Pass C(++) compiler to go build.
diff: import collections
  import copy
  import drake
  +import drake.cxx
  import os
  import subprocess
  @@ -169,7 +170,8 @@ class Toolkit:
  path = None,
  root = None,
  os = None,
  - arch = None):
  + arch = None,
  + cxx_toolkit = None):
  """
  Create a toolkit or clone an existing one.
  @@ -210,6 +212,7 @@ class Toolkit:
  self.__root = root
  self.__version = None
  self.__env = False
  + self.__cxx_toolkit = cxx_toolkit
  if self.go is None:
  raise Exception('go executable is undefined. Check its installation')
  try:
  @@ -257,6 +260,10 @@ class Toolkit:
  if self.__env is not False:
  return self.__env
  self.__env = dict(os.environ)
  + self.__env['CGO_ENABLED'] = '1'
  + if self.__cxx_toolkit:
  + self.__env['CC'] = self.__cxx_toolkit.c
  + self.__env['CXX'] = self.__cxx_toolkit.cxx
  for k in Toolkit.properties:
  v = getattr(self, k)
  if v is not None:
  