Linux evdev controller backend fixes (and misc linux fixes)

[hcorion]

This PR includes

• Improved evdev controller detection, this fixes detection of the Steam Controller when used with sc-controller
• Adds joystick squircling, which makes running in Persona 5 nicer, same implementation as xinput and dualshock 4 backends.
• Adds a quick fix for developer builds and standard linux builds (for example, the Arch AUR), to no longer give experimental build warning.
• Updates ffmpeg again, to fix the issue here: http://www.emunewz.net/forum/showthread.php?tid=174368&pid=285178#pid285178 again.

Requesting review from @kirbyfan64 @Kozek

Requesting testing from @gasinvein @gadzook @ghost and @toccata10

---

pre-compiled AppImage link for testing: https://transfer.sh/pwI71/eb22e4e-178_linux64
(will expire in 14 days)
To run AppImage:

wget https://transfer.sh/pwI71/eb22e4e-178_linux64
chmod a+x ./eb22e4e-178_linux64
./eb22e4e-178_linux64

[refi64]

/usr/bin/ld: mux.o: access beyond end of merged section (19645)

What the heck?

[hcorion]

IKR, the weird thing is that it failed before I updated the ffmpeg submodule, but only in the gcc build.

[hcorion]

Ok, turns out the weird linking issue was a bug in binutils 2.26 linking against libraries (namely, the updated ffmpeg ones) compiled with a newer gcc, so we have to use a newer binutils version for the build.

[gasinvein]

@hcorion, gamepad with sc-controller works, but rpcs3 log is filled with lines E Unknown joystick #2 event 2, and also this seems like emulation is slowed down.

[hcorion]

Hmmm, can you upload your RPCS3.log @gasinvein ? (It's at ~/.config/rpcs3/RPCS3.log), looks like my joystick detector is faulty.

[refi64]

I think I know the problem. The JOYSTICK_BUSTYPE macro is actually just BUS_USB, so all USB devices are getting picked up as joysticks.

A "proper" solution would be to use libudev, which invokes callbacks with arguments that can be used to determine if the input device is a joystick (it would also add room for hot plugging). However, as a temporary fix, we could mimic what SDL does. Off the top of my head, I think a libevdev equivalent would be:

if (libevdev_has_event_type(dev, EV_KEY) &&
    libevdev_has_event_code(dev, EV_ABS, ABS_X) &&
    libevdev_has_event_code(dev, EV_ABS, ABS_Y))
{
    // It's a joystick.
}