s3-waf: work around broken cups-config --libs output

s3-waf: work around broken cups-config --libs output

"cups-config --libs" returns -lgssapi_krb5, which ties it to the
system krb5 library. It should get this via the indirect dependency of
libcups.so instead.

Work around this by using 'cups' as the library and avoid using --libs
in the cups-config command

Pair-Programmed-With: Andrew Bartlett <abartlet@samba.org>