cups: fix libdir handling

Cups uses /usr/lib64 if the HOST has it, no matter if it is cross compiling
for a 32bit arch, breaking the build for stuff looking in /usr/lib.

The fix of commit edd2716c didn't work, it would just force /usr/lib64 if
the target is 64 bit.  Instead, force installation in /usr/lib regardless
of the host.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>