-PKG_CHECK_MODULES([SDL_image], [SDL_image])
-PKG_CHECK_MODULES([libpng], [libpng12])
+
+# These are only needed for the demo app.
+with_demo_app=yes
+PKG_CHECK_MODULES([SDL_image], [SDL_image], [], [with_demo_app=no; AC_MSG_WARN([SDL_image not found, demo program will not be built])])
+PKG_CHECK_MODULES([libpng], [libpng12], [], [with_demo_app=no; AC_MSG_WARN([libpng12 not found, demo program will not be built])])
+
+AC_SUBST([with_demo_app])