Most peripherals, such as a joystick, are accessed through operating system services whose implementation may be equivalent, yet very different, under the various operating systems. FlightGear currently supports Windows, FreeBSD, Linux, Solaris, MacOS, Irix and OS-X. For those services which are common across most video games, the independent project PLIB offers a simple API that acts as a Portable Library.
Compared to Windows, MacOS and the Unix's, the various distributions and releases of Linux-based operating systems are very similar. There are important differences, most of which cause problems when trying to build and test
PLIB, so these rarely impact FlightGear directly.
Any Linux user can download the source, compile it and safely expect it to run provided OpenGL works.