Introduce and use LDAP_GCCATTR() = __attributes__() in gcc

clients/fax500/main.c

clients/gopher/go500.c

clients/gopher/go500gw.c

clients/mail500/main.c

clients/tools/ldapmodify.c

clients/ud/ud.h

include/ac/setproctitle.h

include/lber_pvt.h

include/ldap_cdefs.h

include/ldap_log.h

libraries/libldap/ldap-int.h

servers/ldapd/proto-ldapd.h

servers/slapd/aclparse.c

servers/slapd/schemaparse.c

servers/slapd/shell-backends/shellutil.h