I found an answer here: http://bugs.gentoo.org/show_bug.cgi?format=multiple&id=192391

And it works! :-)

It changes APR_XtOffsetOf to APR_OFFSETOF in the .c file.

Patch which solves the problem:

--- mod_auth_mysql.c.orig       2007-09-13 09:36:14.000000000 +0200
+++ mod_auth_mysql.c    2007-09-13 09:41:49.000000000 +0200
@@ -589,87 +589,87 @@
 static
 command_rec mysql_auth_cmds[] = {
        AP_INIT_TAKE1("AuthMySQLHost", ap_set_string_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlhost),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlhost),
        OR_AUTHCFG, "mysql server host name"),

        AP_INIT_TAKE1("AuthMySQLPort", ap_set_int_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlport),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlport),
        OR_AUTHCFG, "mysql server port number"),

        AP_INIT_TAKE1("AuthMySQLSocket", ap_set_string_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlsocket),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlsocket),
        OR_AUTHCFG, "mysql server socket path"),

        AP_INIT_TAKE1("AuthMySQLUser", ap_set_string_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqluser),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqluser),
        OR_AUTHCFG, "mysql server user name"),

        AP_INIT_TAKE1("AuthMySQLPassword", ap_set_string_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlpasswd),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlpasswd),
        OR_AUTHCFG, "mysql server user password"),

        AP_INIT_TAKE1("AuthMySQLDB", ap_set_string_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlDB),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlDB),
        OR_AUTHCFG, "mysql database name"),

        AP_INIT_TAKE1("AuthMySQLUserTable", ap_set_string_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlpwtable),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlpwtable),
        OR_AUTHCFG, "mysql user table name"),

        AP_INIT_TAKE1("AuthMySQLGroupTable", ap_set_string_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlgrptable),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlgrptable),
        OR_AUTHCFG, "mysql group table name"),

        AP_INIT_TAKE1("AuthMySQLNameField", ap_set_string_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlNameField),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlNameField),
        OR_AUTHCFG, "mysql User ID field name within User table"),

        AP_INIT_TAKE1("AuthMySQLGroupField", ap_set_string_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlGroupField),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlGroupField),
        OR_AUTHCFG, "mysql Group field name within table"),

        AP_INIT_TAKE1("AuthMySQLGroupUserNameField", ap_set_string_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec,
mysqlGroupUserNameField),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlGroupUserNameField),
        OR_AUTHCFG, "mysql User ID field name within Group table"),

        AP_INIT_TAKE1("AuthMySQLPasswordField", ap_set_string_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlPasswordField),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlPasswordField),
        OR_AUTHCFG, "mysql Password field name within table"),

        AP_INIT_TAKE1("AuthMySQLPwEncryption", ap_set_string_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlEncryptionField),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlEncryptionField),
        OR_AUTHCFG, "mysql password encryption method"),

        AP_INIT_TAKE1("AuthMySQLSaltField", ap_set_string_slot,
-       (void*) APR_XtOffsetOf(mysql_auth_config_rec, mysqlSaltField),
+       (void*) APR_OFFSETOF(mysql_auth_config_rec, mysqlSaltField),
        OR_AUTHCFG, "mysql salfe field name within table"),

 /*     AP_INIT_FLAG("AuthMySQLKeepAlive", ap_set_flag_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlKeepAlive),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlKeepAlive),
        OR_AUTHCFG, "mysql connection kept open across requests if On"),
 */
        AP_INIT_FLAG("AuthMySQLAuthoritative", ap_set_flag_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlAuthoritative),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlAuthoritative),
        OR_AUTHCFG, "mysql lookup is authoritative if On"),

        AP_INIT_FLAG("AuthMySQLNoPasswd", ap_set_flag_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlNoPasswd),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlNoPasswd),
        OR_AUTHCFG, "If On, only check if user exists; ignore password"),

        AP_INIT_FLAG("AuthMySQLEnable", ap_set_flag_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlEnable),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlEnable),
        OR_AUTHCFG, "enable mysql authorization"),

        AP_INIT_TAKE1("AuthMySQLUserCondition", ap_set_string_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlUserCondition),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlUserCondition),
        OR_AUTHCFG, "condition to add to user where-clause"),

        AP_INIT_TAKE1("AuthMySQLGroupCondition", ap_set_string_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlGroupCondition),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlGroupCondition),
        OR_AUTHCFG, "condition to add to group where-clause"),

        AP_INIT_TAKE1("AuthMySQLCharacterSet", ap_set_string_slot,
-       (void *) APR_XtOffsetOf(mysql_auth_config_rec, mysqlCharacterSet),
+       (void *) APR_OFFSETOF(mysql_auth_config_rec, mysqlCharacterSet),
        OR_AUTHCFG, "mysql character set to be used"),

   { NULL }

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