Briefly: keep it NO if you do not have any issues building MySQL.
Usually DirectAdmin ships customized packages for each Operating System. You should keep mysql_force_compile NO to use them (this is recommended!).
The mysql_force_compile is used in cases when you try to build it and it fails because it can't find a customized package. For example it may happen on Debian. In such cases you set the setting to YES and custombuild will therefore download a generic mysql package and compile it. Here is more info: