Platform: add helpful error messages to application test build files.

src/Magnum/Platform/Test/CMakeLists.txt

@@ -85,6 +85,9 @@ if(MAGNUM_WITH_GLFWAPPLICATION)
     # HiDPi.manifest not needed, as GLFW sets that on its own
     target_link_libraries(PlatformGlfwApplicationTest PRIVATE MagnumGlfwApplication Corrade::Main)
     # Window icon loading
+    if(NOT MAGNUM_WITH_TRADE)
+        message(FATAL_ERROR "GlfwApplication tests need the Trade library enabled")
+    endif()
     target_sources(PlatformGlfwApplicationTest PRIVATE ${Platform_RESOURCES})
     target_link_libraries(PlatformGlfwApplicationTest PRIVATE MagnumTrade)
     if(CORRADE_TARGET_APPLE)
@@ -112,6 +115,9 @@ if(MAGNUM_WITH_SDL2APPLICATION)
     endif()
     target_link_libraries(PlatformSdl2ApplicationTest PRIVATE MagnumSdl2Application Corrade::Main)
     # Window icon loading
+    if(NOT MAGNUM_WITH_TRADE)
+        message(FATAL_ERROR "GlfwApplication tests need the Trade library enabled")
+    endif()
     if(NOT CORRADE_TARGET_EMSCRIPTEN)
         target_sources(PlatformSdl2ApplicationTest PRIVATE ${Platform_RESOURCES})
         target_link_libraries(PlatformSdl2ApplicationTest PRIVATE MagnumTrade)