mk: Switch KRB5_DEFAULT default to mit-krb5.
More OS include this than heimdal, and for OS that do not provide
either, building the pkgsrc version is a lot more portable.
Retain heimdal as the default on NetBSD and FreeBSD.
gcc14-darwin: Back to dynamic PLIST.
Can't use static like we do on SunOS as this retains fixincludes which
differs across OS releases, and that's just too much effort.