Added workaround for legacy MinGW.

When building on legacy MinGW, WINVER and UNICODE need to be defined
before the inclusion of stddef.h (by glfw3.h), which is itself included
before win32_platform.h.

CMakeLists.txt

@@ -163,6 +163,13 @@ if (_GLFW_WIN32)
     if (GLFW_USE_OPTIMUS_HPG)
         set(_GLFW_USE_OPTIMUS_HPG 1)
     endif()
+
+    # HACK: When building on MinGW, WINVER and UNICODE need to be defined before
+    # the inclusion of stddef.h (by glfw3.h), which is itself included before
+    # win32_platform.h.  We define them here until a saner solution can be found
+    # NOTE: MinGW-w64 and Visual C++ do /not/ need this hack.
+    add_definitions(-DUNICODE)
+    add_definitions(-DWINVER=0x0501)
 endif()
 
 #--------------------------------------------------------------------

README.md

@@ -205,6 +205,7 @@ See the [GLFW documentation](http://www.glfw.org/docs/latest/).
 ## Changelog
 
  - [Win32] Bugfix: `_WIN32_WINNT` was not set to Windows XP or later
+ - [Win32] Bugfix: Legacy MinGW needs `WINVER` and `UNICODE` before `stddef.h`
  - [X11] Bugfix: Events for mouse buttons 4 and above were not reported