Fix meson pkgconfig

[mgautierfr]

Let's meson's module pkg-config generate totally our libzim.pc instead of trying to give smart values.

[kelson42]

@mgautierfr OK, but why?

[mgautierfr]

This is somehow explained in commits messages. Here a better explanation:

• When using dependency from subproject compilation, we don't have a pc file. We must give the link options directly in the libzim.pc. We were always (explicitly) setting that we were dependending of other .pc files. Meson handle this correctly and set the options as needed so let's use it.
• Same for (removed) extra_link_args, we have to pass the options in the libzim.pc. Meson handle that correctly when using a (internal) declared dependency.

All of this is needed to pass the correctly linking argument to other project depending of libzim (especially on Windows).