Win32: Add warning when option will have no effect

The GPU driver only looks in the executable for the symbol requesting
the high-performance GPU, so enabling them when buidling GLFW as a DLL
will have no effect.

src/win32_init.c

@@ -39,6 +39,10 @@ static const GUID _glfw_GUID_DEVINTERFACE_HID =
 
 #if defined(_GLFW_USE_HYBRID_HPG) || defined(_GLFW_USE_OPTIMUS_HPG)
 
+#if defined(_GLFW_BUILD_DLL)
+ #warning "These symbols must be exported by the executable and have no effect in a DLL"
+#endif
+
 // Executables (but not DLLs) exporting this symbol with this value will be
 // automatically directed to the high-performance GPU on Nvidia Optimus systems
 // with up-to-date drivers