rev 1.399 (dec 5) of bsd.own.mk added the variable USE_INET6, based on
MKINET6.  Its value is lowercase yes/no.  pkgsrc also uses USE_INET6
based on _OPSYS_HAS_INET6, from bsd.prefs.mk.

The problem is that pkgsrc expects the value (yes/no) to be uppercase
and (at least) the hand-full of Makefiles that I've checked only check
for uppercase.

I have not checked the src/ tree to see exactly where/how it is used.

Any suggestions on how to correct this?  I thought that bsd.prefs.mk
could use :tu to re-set USE_INET6, but that makes the variable recursive
or I just misunderstand the syntax or the order of evaluation occuring
here.