[lmi] Upgrade gcc now?

[Greg Chicares]

Vadim--We're thinking of upgrading gcc now, and seek your advice. This
might be an especially good time for an upgrade, because before long
Kim will get switched from msw-7 to msw-10, and it's possible that her
current installation at that time will be migrated, but her ability to
upgrade it further might be blocked for a while (because access to the
internet is ever more aggressively blocked, and crazy things happen
when the corporation changes the OS version).

Right now Kim and I both use gcc-4.9.x . She can pick any version that
the MinGW-w64 project has released:
  https://sourceforge.net/projects/mingw-w64/files/mingw-w64/
7.1, 6.3, 6.2, and 5.4 seem to be the likely candidates. So far, I've
been using only debian packages, and if I stay on that track I could
create a debian-testing chroot with 6.3.0 (same gcc version as for
debian unstable today). Alternatively, I could try using a package
from MinGW-w64 directly, which might or might not be easy, so that
leads me to my first question: do you think 7.1 is so much better
than 6.3 that I should try to do this?

Here's my other question: which flavor(s) should we plan to use? If
we're going to upgrade anyway, we may as well fetch both i686 and
x86_64 versions: I'm pretty sure nobody is running lmi on a 32-bit OS
any more, so we should experiment with a 64-bit build. The question
then comes down to: which exception models? For i686, I suppose sjlj
is still the only one we can expect to work, because we need to throw
exceptions across the DLL and kernel boundaries. But for x86_64, will
seh avoid that problem, or will we still need sjlj?

IOW, of the files offered here:
  https://sourceforge.net/projects/mingw-w64/files/mingw-w64/
today we're using
  i686-4.9.1-release-win32-sjlj-rt_v3-rev3.7z
but for 6.3.0 I think we want
  i686-6.3.0-release-win32-sjlj-rt_v5-rev1.7z
and one of these two
  x86_64-6.3.0-release-win32-sjlj-rt_v5-rev1.7z
  x86_64-6.3.0-release-win32-seh-rt_v5-rev1.7z
where AFAICT "win32" selects msw rather than posix threads.