HEADS UP: switch to GCC 6.4 coming

[matthew green <mrg%eterna.com.au@localhost>]

hi folks.


i'll be switching the i386, sparc and sparc64 ports to GCC 6.4.0
this weekend.  the alpha, mips, and powerpc ports are also mostly
ready to go, and arm could with some more testing.  unlisted ports
need testing, but should build.  (most ports haven't had their
full release build tested, which is the main reason most of these
are not ready to switch right now.)

this will cause build failures that need tools/gcc objdir and the
destdir to be cleaned up, if not more.  please clean those dirs
and try again, and if more failure, try a completely clean build.

for now, you can force GCC 5 builds with "HAVE_GCC=5".

as always, please send-pr any issues you find.  thanks!


.mrg.