This allows Xvnc to build with xorg-server 1.20.7, which requires OS
layers to implement a ddxInputThreadInit function when configured with
--enable-input-thread (the default).
relevant xorg-server commit: e3f26605d85d987da434640f52646d728f1fe919
When used with -fg we expect the startup script to remain running until
the session is over. This will not happen if the session command is put
in the background using &.