Win32: Fix symbol redefinition warnings

When both GLFW_INCLUDE_VULKAN and VK_USE_PLATFORM_WIN32_KHR were
defined, the GLFW header would define replacement versions of APIENTRY
and WINGDIAPI /before/ including the Vulkan header, which would include
windows.h, which (justifiably) defines APIENTRY and WINGDIAPI blindly.

Fixes #1524.

README.md

@@ -121,6 +121,8 @@ information on what to include when reporting a bug.
  - Disabled tests and examples by default when built as a CMake subdirectory
  - Bugfix: The CMake config-file package used an absolute path and was not
    relocatable (#1470)
+ - [Win32] Bugfix: `GLFW_INCLUDE_VULKAN` plus `VK_USE_PLATFORM_WIN32_KHR` caused
+   symbol redefinition (#1524)
  - [X11] Bugfix: The CMake files did not check for the XInput headers (#1480)
  - [X11] Bugfix: Key names were not updated when the keyboard layout changed
    (#1462,#1528)

include/GLFW/glfw3.h

@@ -96,6 +96,25 @@ extern "C" {
  #define _WIN32
 #endif /* _WIN32 */
 
+/* Include because most Windows GLU headers need wchar_t and
+ * the macOS OpenGL header blocks the definition of ptrdiff_t by glext.h.
+ * Include it unconditionally to avoid surprising side-effects.
+ */
+#include <stddef.h>
+
+/* Include because it is needed by Vulkan and related functions.
+ * Include it unconditionally to avoid surprising side-effects.
+ */
+#include <stdint.h>
+
+#if defined(GLFW_INCLUDE_VULKAN)
+  #include <vulkan/vulkan.h>
+#endif /* Vulkan header */
+
+/* The Vulkan header may have indirectly included windows.h (because of
+ * VK_USE_PLATFORM_WIN32_KHR) so we offer our replacement symbols after it.
+ */
+
 /* It is customary to use APIENTRY for OpenGL function pointer declarations on
  * all platforms.  Additionally, the Windows OpenGL header needs APIENTRY.
  */
@@ -122,17 +141,6 @@ extern "C" {
  #define GLFW_CALLBACK_DEFINED
 #endif /* CALLBACK */
 
-/* Include because most Windows GLU headers need wchar_t and
- * the macOS OpenGL header blocks the definition of ptrdiff_t by glext.h.
- * Include it unconditionally to avoid surprising side-effects.
- */
-#include <stddef.h>
-
-/* Include because it is needed by Vulkan and related functions.
- * Include it unconditionally to avoid surprising side-effects.
- */
-#include <stdint.h>
-
 /* Include the chosen OpenGL or OpenGL ES headers.
  */
 #if defined(GLFW_INCLUDE_ES1)
@@ -211,10 +219,6 @@ extern "C" {
 
 #endif /* OpenGL and OpenGL ES headers */
 
-#if defined(GLFW_INCLUDE_VULKAN)
-  #include <vulkan/vulkan.h>
-#endif /* Vulkan header */
-
 #if defined(GLFW_DLL) && defined(_GLFW_BUILD_DLL)
  /* GLFW_DLL must be defined by applications that are linking against the DLL
   * version of the GLFW library.  _GLFW_BUILD_DLL is defined by the GLFW