Go 1.10 does not build on NetBSD 6

[Benny Siegert <bsiegert%gmail.com@localhost>]

As seen here:
http://nyftp.netbsd.org/pub/pkgsrc/packages/reports/2018Q2/NetBSD-6.0-x86_64/20180703.0758/go-1.10.3/build.log

How much do we care?

As far as I remember, the minimum NetBSD version that upstream
considers supported (-ish) is 7.1, with all development and testing
done on current or the 8 branch.

I imagine (but I have not tested) that many Go packages in the tree
will fail to build with Go 1.4. One option would be to introduce
lang/go18 (or whatever the last version working on NetBSD was) and add
a switch to use that by default. However, pulling that up into the
stable branch would be icky.

-- 
Benny