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).
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