Wayland: Explain what to do when the build fails

That would be a helpful thing to do.

CMakeLists.txt

@@ -29,7 +29,7 @@ endif()
 
 if (DEFINED GLFW_USE_WAYLAND AND UNIX AND NOT APPLE)
     message(FATAL_ERROR
-        "GLFW_USE_WAYLAND has been removed; set the GLFW_BUILD_WAYLAND and GLFW_BUILD_X11 options")
+        "GLFW_USE_WAYLAND has been removed; delete the CMake cache and set GLFW_BUILD_WAYLAND and GLFW_BUILD_X11 instead")
 endif()
 
 cmake_dependent_option(GLFW_BUILD_WIN32 "Build support for Win32" ON "WIN32" OFF)