Start using the fact that SUBDIR.yes is added to SUBDIR
and move from the pattern of:

.if ${MK_FOO} != "no"
SUBDIR+= bar
.endif

to

SUBDIR.${MK_FOO}+= bar

since we know that MK_FOO is always either yes or no and the latter
form is easier to follow and much shorter. Various exception to this
pattern dealt with on an ah-hoc basis.

Discussed on arch@ a while ago.