allow cxx flag to be full path to compiler
fix to use user specified compiler
check specified compiler is g++
Merge branch 'master' into allow_cxx_flag_full_path

Conflicts:
	theano/gof/cmodule.py
fix error message
improve waring message
Merge pull request #2242 from cocu/allow_cxx_flag_full_path

allow cxx flag to be full path to compiler