CUDA 9.1 requires the following changes to be used with gcc versions > 6

https://carterturn.com/h/Guides/NVIDIA%20CUDA%209.1%20with%20GCC%207.2

After updating my system to Fedora 27, I was pleased to find that I had a shiny new GCC version, GCC 7.2.1 (I enjoy new compilers too much). Unfortuntely, the most recent version of NVIDIA's CUDA, CUDA 9.1, which I also enjoy using, is only compatible with GCC 6. Here, I will document my workaround for this, which allows one to use most (seems like all) of CUDA's features with GCC 7.2.1.
CUDA 9.1

First, install CUDA 9.1. When installing, add the --override flag to the installer and say yes when it asks to install in an unsupported configuration (that is the entire point of this, right?).

Modify host_config.h
As usual, we need to comment out the section of NVIDIA's host_config.h that prevents us from using newer GCC versions. In a slight twist from last time, we need to modify /usr/local/cuda/include/crt/host_config.h (Note that the /usr/local/cuda part depends on where you chose to install during the installation). Comment out the line

#error – unsupported GNU version! gcc versions later than 6 are not supported!

For me, that was line 121.

Modify floatn.h
For some reason, nvcc does not like 128 bit floats, so we need to modify /usr/include/bits/floatn.h. The easiest way to do this (e.g. the way that involves opening as few files as possible) is to add

#ifdef _CUDACC_
 #define __HAVE_FLOAT128 0
 #endif

just after the section of floatn.h that defines __HAVE_FLOAT128. In my copy, this was line 37.

So far, the only error I am getting is related to having an outdated driver, so I think this works.