Added threading library to link dependencies.

Since the GLX backend now uses explicit pthread calls, linking needs to
be explicit as well.

CMakeLists.txt

@@ -162,7 +162,10 @@ if (_GLFW_X11)
 
     # Set up library and include paths
     list(APPEND glfw_INCLUDE_DIRS ${X11_X11_INCLUDE_PATH})
-    list(APPEND glfw_LIBRARIES ${X11_X11_LIB})
+    list(APPEND glfw_LIBRARIES ${X11_X11_LIB} ${CMAKE_THREAD_LIBS_INIT})
+    if (UNIX AND NOT APPLE)
+        list(APPEND glfw_LIBRARIES ${RT_LIBRARY})
+    endif()
 
     # Check for XRandR (modern resolution switching and gamma control)
     if (NOT X11_Xrandr_FOUND)