Meson: fix preprocessor directives.

When using #ifdef in your code (vs #if), the C preprocessor doesn't check the value of the macro, only whether it is defined at all.

By using conf.set10(), the macros were defined, whether the boolean values in the meson build file were false or not. So, to fix this, you either have to start using #if, or you make sure you use conf.set() instead.

This fixes the flatpak build of GNOME Contacts (which turns off udev support).