[Gm2] backporting complete on gcc-6.4.0

[Gaius Mulley]

Hello,

apologies for the delay - but I think that the backport of the svn-trunc
gm2 changes from October 2017 though to January 2018 is complete.  The
major user level improvements are -fsoft-check-all will now detect
floating point nans at runtime and partial integer overflow
(add/sub/negate).

The gcc-6.4.0 tarball should be created on floppsie automatically in an
hour.  If it builds successfully on a variety of machines I'll generate
a deb package and re-release gm2 for the gcc-6.4.0 branch (gm2-1.6.4)

The testsuite in gm2-1.6.4 reports:

      === gm2 Summary ===

# of expected passes    10736

on a 64bit amd debian stretch machine

regards,
Gaius