Improve CHECK_LIB interaction with CHECK_PKG

Improve CHECK_LIB interaction with CHECK_PKG

When checking for shared libraries, only name the target library
if it was not previously discoverd by pkg-config --libs and now
available from uselib_store. This avoids using both sources of
information which results in the library being named twice on
the command line.

Once the library is confirmed by CHECK_LIB, append the library if
not already present, to avoid dropping libraries that were
previously discovered by CHECK_PKG.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15623

Signed-off-by: Earl Chew <earl_chew@yahoo.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>