Use MK_* to prevent debug files if possible.

A longstanding bug with the base /usr/share/mk files is that /etc/src.conf
*does* get pulled in when building ports which use /usr/share/mk. This despite
the documentation saying it is only used for "the FreeBSD tree". If users
have a WITH_DEBUG_FILES=yes in their src.conf then the previous
WITHOUT_DEBUG_FILES=yes would hit an error. Fixing pulling in src.conf is
risky. MK_* overrides work fine here after recent stable/9 and stable/10
updates.

If you run into the WITH_ vs WITHOUT_ problem on a release build then you can
protect your src.conf value with something like:

.if !${.CURDIR:M*ports*}
WITH_DEBUG_FILES=yes
.endif

PR: D6271
With hat: portmgr
Sponsored by: EMC / Isilon Storage Division