guard against trying to compile SU3-specific code when Nc ≠ 3

[edbennett]

Currently it's not possible to make install Grid for any --enable-Nc ≠ 3 without either manually removing files or editing Makefiles, as certain examples implicitly rely on functions only defined for Nc==3.

This PR guards these examples such that they compile successfully (albeit to files that only print a warning that they don't work) when Nc ≠ 3.