Fix linker error in EntryPoint if ffmpeg is missing due to missing DISABLE_VIDEO in vcmiclient target

clientapp/CMakeLists.txt

@@ -56,6 +56,11 @@ if(WIN32)
 	endif()
 	target_compile_definitions(vcmiclient PRIVATE WINDOWS_IGNORE_PACKING_MISMATCH)
 
+	if(NOT ffmpeg_LIBRARIES)
+		target_compile_definitions(vcmiclient PRIVATE DISABLE_VIDEO)
+	endif()
+
+
 	# TODO: very hacky, find proper solution to copy AI dlls into bin dir
 	if(MSVC)
 		add_custom_command(TARGET vcmiclient POST_BUILD