Fix warnings about obsolete/deprecated features reported by modern autotools. Also updates third-party m4 macros.
In drv_mysql.c do not assume MYSQL_OPT_SSL_MODE to be a preprocessor define. It is an mysql_option enum value with all MySQL versions. Instead, do a configure-time check if that value is defined in mysql.h.
Add a configure check for <server/mysql_version.h> which appears to be the most portable way to fix MariaDB builds (see MDEV-13773).
