Fix some bugs in variable pattern substitution (e.g. $(VAR:A=B)),

Fix some bugs in variable pattern substitution (e.g. $(VAR:A=B)),
reported by Markus Mauhart <qwe123@chello.at>.  One was a simple typo; to
fix the other we call patsubst_expand() for all instances of variable
substitution, even when there is no '%'.  We used to call subst_expand()
with a special flag set in the latter case, but it didn't work properly
in all situations.  Easier to just use patsubst_expand() since that's
what it is.