Linux: Add support for kernels < v2.6.39

Broke build for me on CentOS-6, which sports a 2.6.32 kernel:
http://www.cpantesters.org/cpan/report/d956d128-0339-11e8-b0d1-b6c4abd39192

CentOS-6 is supported till 2020-11-30.

Closes #1196.

README.md

@@ -233,6 +233,8 @@ information on what to include when reporting a bug.
 - [X11] Bugfix: Selection I/O reported but did not support `COMPOUND_TEXT`
 - [X11] Bugfix: Latin-1 text read from selections was not converted to UTF-8
 - [X11] Bugfix: NVidia EGL would segfault if unloaded before closing the display
+- [Linux] Added workaround for missing `SYN_DROPPED` in pre-2.6.39 kernel
+          headers (#1196)
 - [Linux] Moved to evdev for joystick input (#906,#1005)
 - [Linux] Bugfix: Event processing did not detect joystick disconnection (#932)
 - [Linux] Bugfix: The joystick device path could be truncated (#1025)

src/linux_joystick.c

@@ -38,6 +38,11 @@
 #include <string.h>
 #include <unistd.h>
 
+#ifndef SYN_DROPPED // < v2.6.39 kernel headers
+// Workaround for CentOS-6, which is supported till 2020-11-30, but still on v2.6.32
+#define SYN_DROPPED 3
+#endif
+
 // Apply an EV_KEY event to the specified joystick
 //
 static void handleKeyEvent(_GLFWjoystick* js, int code, int value)