Currently, if the location of the Cups headers is passed to configure via the --with-cups or --with-cups-inlcude option, the locations are not checked to really contain the required Cups header files. This will lead to a compilation error, if there are no Cups header files in the default include path.

Even worse, if the user wants to specify special versions of the Cups header files via the --with-cups or --with-cups-inlcude options but mis-types the path while there exist Cups header files in the standard include path, the standard headers will be used silently (because the location given by --with-cups or --with-cups-inlcude isn't checked and rejected.)