Background: Some distributions use tmpfs for mounting /tmp by default, which has some advantages, but brings also new issues. Fedora started using tmpfs on /tmp in version 18 for example. If not configured otherwise in my.cnf, MySQL/MariaDB uses system's constant P_tmpdir expanded to /tmp on Linux. This can introduce some problems with limited space in /tmp and also some data loss in case of replication slave .
In case distributions would like to use /var/tmp, which should be better for MySQL/MariaDB purposes, then we have to patch the source or change tmpdir option in my.cnf, which is however not updated in case it has already existed.
Thus, it would be useful to be able to specify default tmpdir path using a configure option, while using P_tmpdir in case it is not defined explicitly. I've created such patch, so feel free to comment or apply it.