NVCC 9.1 + GCC-8 workaround

[Mo Zhou]

Hi anbe and nvidia team,

During debconf Ximin and I have figured out a workaround to make NVCC
work with GCC-8. Using this workaround I have successfully compiled
src:caffe-contrib.

CXXFLAGS += -std=c++03 -D__GNUC__=8

We need to make GCC-8 disguise to be GCC-6, and downgrade CXX standard
to c++03. I don't know whether this works for other packages, but at
least src:caffe-contrib will get rid of GCC-6 for the next upload.