Add conditional compilation for platform units

This is a step towards being able to compile GLFW manually without
needing to duplicate a lot of platform- or OS-specific logic.

src/wl_init.c

@@ -28,6 +28,8 @@
 
 #include "internal.h"
 
+#if defined(_GLFW_WAYLAND)
+
 #include <errno.h>
 #include <limits.h>
 #include <linux/input.h>
@@ -392,7 +394,7 @@ GLFWbool _glfwConnectWayland(int platformID, _GLFWplatform* platform)
         _glfwGetKeyScancodeWayland,
         _glfwSetClipboardStringWayland,
         _glfwGetClipboardStringWayland,
-#if defined(__linux__)
+#if defined(_GLFW_LINUX_JOYSTICK)
         _glfwInitJoysticksLinux,
         _glfwTerminateJoysticksLinux,
         _glfwPollJoystickLinux,
@@ -791,3 +793,5 @@ void _glfwTerminateWayland(void)
     _glfw_free(_glfw.wl.clipboardString);
 }
 
+#endif // _GLFW_WAYLAND
+