Optimization bug when byte compiling with gcc 5.3.0 on windows

I've started to look into building R for Windows using MSYS2 as both the
build environment and tools + libraries provider (where possible).

Thanks for your reply, and for the patches.

Last time I had a look at this (a few months ago) another problem was
that mingw-w64 v4 was giving different numeric output for some of the
tests in r-base. If I recall correctly, some eigen vectors had their
direction flipped (negative values became positive and vice versa).
Did you notice anything of this kind when running 'make check' and
'make check recommended' ? It is important to us that numeric results
are reproducible between versions of R.