GLFW: Fix build error on Linux < v2.6.39

Cherry-pick from upstream glfw/glfw#1196
Found in CPANtesters test of Alien::raylib:
http://www.cpantesters.org/cpan/report/d956d128-0339-11e8-b0d1-b6c4abd39192

src/external/glfw/src/linux_joystick.c

@@ -38,6 +38,10 @@
 #include <string.h>
 #include <unistd.h>
 
+#ifndef SYN_DROPPED // < 2.6.39 kernel headers
+#define SYN_DROPPED 3
+#endif
+
 // Apply an EV_KEY event to the specified joystick
 //
 static void handleKeyEvent(_GLFWjoystick* js, int code, int value)