php: set mysql socket path if mysql[i] or pdo_mysql support is enabled

PHP tries to discover the mysql default socket path during configure
phase by probing the file system:
https://github.com/php/php-src/blob/cf3b852109a88a11370d0207cd3b72a53b6a64c3/ext/mysqli/config.m4#L4

This obviously fails to discover /run/mysqld/mysqld.sock, which is being
used (hardcoded) across all MySQL flavours.

This leads to PHP having no mysql socket path set for the mysql[i]
extensions, and `/tmp/mysql.sock` set for pdo_mysql,
meaning one currently has to manually configure and set it in php.ini.

Luckily, PHP supports setting that path via
`--with-mysql-sock=/run/mysqld/mysqld.sock` during configure phase,
so let's do this as soon as one of the three modules is enabled.