Fix APIENTRY from windows.h being undefined

Some extension loader headers include windows.h.  If they were included
before glfw3.h, glfw3native.h would leave APIENTRY undefined.  This adds
the GLFW_APIENTRY_DEFINED macro to signal when GLFW "owns" APIENTRY and
may undefine it.

Fixes #1062.

include/GLFW/glfw3.h

@@ -105,6 +105,7 @@ extern "C" {
  #else
   #define APIENTRY
  #endif
+ #define GLFW_APIENTRY_DEFINED
 #endif /* APIENTRY */
 
 /* Some Windows OpenGL headers need this.

include/GLFW/glfw3native.h

@@ -86,7 +86,10 @@ extern "C" {
  // This is a workaround for the fact that glfw3.h needs to export APIENTRY (for
  // example to allow applications to correctly declare a GL_ARB_debug_output
  // callback) but windows.h assumes no one will define APIENTRY before it does
- #undef APIENTRY
+ #if defined(GLFW_APIENTRY_DEFINED)
+  #undef APIENTRY
+  #undef GLFW_APIENTRY_DEFINED
+ #endif
  #include <windows.h>
 #elif defined(GLFW_EXPOSE_NATIVE_COCOA)
  #include <ApplicationServices/ApplicationServices.h>