Win32: Fix some of the errors for Null-only on VS

This workaround is needed whenever windows.h is included after glfw3.h.

src/win32_thread.h

@@ -25,6 +25,11 @@
 //
 //========================================================================
 
+// This is a workaround for the fact that glfw3.h needs to export APIENTRY (for
+// example to allow applications to correctly declare a GL_KHR_debug callback)
+// but windows.h assumes no one will define APIENTRY before it does
+#undef APIENTRY
+
 #include <windows.h>
 
 #define GLFW_WIN32_TLS_STATE            _GLFWtlsWin32     win32;

src/win32_time.h

@@ -25,6 +25,11 @@
 //
 //========================================================================
 
+// This is a workaround for the fact that glfw3.h needs to export APIENTRY (for
+// example to allow applications to correctly declare a GL_KHR_debug callback)
+// but windows.h assumes no one will define APIENTRY before it does
+#undef APIENTRY
+
 #include <windows.h>
 
 #define GLFW_WIN32_LIBRARY_TIMER_STATE  _GLFWtimerWin32   win32;