GCC 7.1.0 & Rtools

Feeling somewhat antsy, I followed Jeroen's instructions and used
niXman's scripts [1] off of the master branch to build gcc 7.1.0 for
x86_64, specifically "x86_64-710-posix-seh-rt_v5-s". It took a dozen
or so hours, but the completed suite compiled "make distribution" on
R-patched_2017-08-27 and passed check-devel. Of course, the external
libraries are still all built with 4.9.3, as is ICU, but I find this
promising.

Jeroen, I think you mentioned having problems with runtime version 4?
This seems to have built runtime version 5, yet I didn't have any
issues.

Something to keep in mind perhaps for the next iteration of Rtools.

Thanks,

Avi

[1] https://github.com/niXman/mingw-builds