cups: move localstatedir from /usr/var to /var

Prior to commit f64a1b85 ("cups:
convert to autotargets"), cups was explicitely configured to
put its modifiable single-machine data under /var.  After that
commit, the --localstatedir was not specified, so it remained at
it's default value, "PREFIX/var" - making cups use "/usr/var".

Using /var to store state files seems preferable to /usr/var,
as /usr might be read-only on some systems.  This patch
re-introduces the --localstatedir specification to build cups
to use /var.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>