#12391 offered to change the default PID file from /var/run/gitea.pid however in discussion it was decided that this could break users of older systems. An alternative was offered that we could make the PID file compile/link time settable.

This PR does this, and changes the name of the setting from CustomPID to simply PIDFile. It also updates the from-source docs to show how to change the compiler settings to do this.

Closes #12391

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Florian Klink <flokli@flokli.de>