Added support for /dev/input/js* on Linux

Added the hint SDL_HINT_LINUX_JOYSTICK_CLASSIC to control whether /dev/input/js* or /dev/input/event* are used as joystick devices

Added the hint SDL_HINT_JOYSTICK_DEVICE to allow the user to specify devices t
hat will be opened in addition to the normal joystick detection

Fixes #1314
Fixes #1727
Fixes #1981
Closes #4727

WhatsNew.txt

@@ -5,6 +5,10 @@ This is a list of major changes in SDL's version history.
 2.0.18:
 ---------------------------------------------------------------------------
 
+Linux:
+* Added the hint SDL_HINT_LINUX_JOYSTICK_CLASSIC to control whether /dev/input/js* or /dev/input/event* are used as joystick devices
+* Added the hint SDL_HINT_JOYSTICK_DEVICE to allow the user to specify devices that will be opened in addition to the normal joystick detection
+
 Android:
 * Added support for audio output and capture using AAudio on Android 8.1 and newer
 

include/SDL_hints.h

@@ -816,6 +816,24 @@ extern "C" {
  */
 #define SDL_HINT_KMSDRM_REQUIRE_DRM_MASTER      "SDL_KMSDRM_REQUIRE_DRM_MASTER"
 
+ /**
+  *  \brief  A comma separated list of devices to open as joysticks
+  *
+  *  This variable is currently only used by the Linux joystick driver.
+  */
+#define SDL_HINT_JOYSTICK_DEVICE "SDL_JOYSTICK_DEVICE"
+
+ /**
+  *  \brief  A variable controlling whether to use the classic /dev/input/js* joystick interface or the newer /dev/input/event* joystick interface on Linux
+  *
+  *  This variable can be set to the following values:
+  *    "0"       - Use /dev/input/event*
+  *    "1"       - Use /dev/input/js*
+  *
+  *  By default the /dev/input/event* interfaces are used
+  */
+#define SDL_HINT_LINUX_JOYSTICK_CLASSIC "SDL_LINUX_JOYSTICK_CLASSIC"
+
  /**
   *  \brief  A variable controlling whether joysticks on Linux adhere to their HID-defined deadzones or return unfiltered values.
   *