configure: fix AC_CHECK_LIB usage
    
    Replace empty string with a single space in the third argument of
    `AC_CHECK_LIB` (`action-if-found`) where applicable.
    
    Empty string in the argument causes `AC_CHECK_LIB` to use the default
    action when a library is found which includes adding the library to `LIBS`,
    which specifies libraries to be linked in every binary and library in the
    project.
    
    This fixes libkrad, liblber, libldap_r and libsss_nss_idmap being linked to
    every binary and library in IPA, even where unused.
    
    https://pagure.io/freeipa/issue/6846
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>