Fixes build against relocated mysql on cvmfs

Fixes future occurrences of ROOT failing to build against MySQL/MariaDB on CVMFS because of unlocatable include/mysql.h file. Diagnosis: mariadb_config, a binary executable, delivers cflags, but stores original install location. Fix: Replace it with mysql_config, which still works as before if it can't find mariadb_configure.

TODO: Look for an official mechanism to fix this. Maybe a mariadb build flag?