Use all LDFLAGS when linking via pkgconfig
Use all LDFLAGS when linking via pkgconfig

As one example, the Ubuntu pkgconfig file for check includes a
`-pthread` switch, which cmake doesn't pass through if we use the
`_LIBRARIES` variable instead of the `_LDFLAGS` variable.

redjack/ipset,dcreager/libcork,redjack/ipset,rogers0/libcork,johnzachary/varon-t,redjack/libcork,dcreager/libcork,rogers0/libcork,tempbottle/libcork,tempbottle/libcork,rogers0/libcork,redjack/libcork,johnzachary/varon-t,johnzachary/varon-t,redjack/libcork,redjack/libcork,redjack/libcork,rogers0/libcork,rogers0/libcork,tempbottle/libcork,redjack/ipset,rogers0/libcork,tempbottle/libcork,redjack/libcork,dcreager/libcork,dcreager/libcork,dcreager/libcork,johnzachary/varon-t,tempbottle/libcork,tempbottle/libcork