Add a variable to help with cross-compilation

This adds a LDAP_HAVE_CONNCB_CROSS variable that will be used instead
of the runtime check for a broken connection callbacks in ldap
library. Set it to yes to assume that connection callbacks are
working.

This allows the project to be configured successfully when
cross-compiling.

src/external/ldap.m4

@@ -69,6 +69,7 @@ AC_CHECK_FUNCS([ldap_control_create ldap_init_fd \
                 ldap_parse_derefresponse_control \
                 ldap_derefresponse_free \
                 ldap_is_ldapc_url])
+AC_ARG_VAR([LDAP_HAVE_CONNCB_CROSS], [A hint about working LDAP connection callbacks for cross-compilation cases])
 AC_CHECK_MEMBERS([struct ldap_conncb.lc_arg],
                  [AC_RUN_IFELSE(
                    [AC_LANG_PROGRAM(
@@ -80,7 +81,9 @@ AC_CHECK_MEMBERS([struct ldap_conncb.lc_arg],
                    [AC_DEFINE([HAVE_LDAP_CONNCB], [1],
                      [Define if LDAP connection callbacks are available])],
                    [AC_MSG_WARN([Found broken callback implementation])],
-                   [])],
+                   [AS_IF([test "x$LDAP_HAVE_CONNCB_CROSS" = xyes],
+                     [AC_DEFINE([HAVE_LDAP_CONNCB], [1],
+                       [Define if LDAP connection callbacks are available])])])],
                  [], [[#include <ldap.h>]])
 
 AC_CHECK_TYPE([LDAPDerefRes],