Cups library fixes

These two commits propose improvements for the cups library:

• cups was compiling the library in a lib64 folder, instead of lib. Although its dependencies checked in both locations, I think that it is preferable, and more consistent with what we do for other components, to use the lib folder.
• More importantly, GTK tried to compile with cups if the library was present in the system, failing the compilation if the includes were not present. This has been explicitly disabled.