Workaround typelib generation for vala libraries

[albfan]

Vala documentation is misleading on generate typelib

This PR tries to offer a working solution meanwhile a better approach is available

relates #2296

[dcbaker]

Would #14782, which gives you a handle for the generated gir file (with correct ordering), be a proper solution to this?

[albfan]

Definitely #14782 is interesting (meson to knowing its build targets), but this tries to workaround a different problem.

if custom target uses an input that is on build dir this errors are shown:

meson setup:

meson.build:36: WARNING: Source item 'build/extensions/Foo-1.0.gir' cannot be converted to File object, because it is a generated file. This will become a hard error in the future.

and building:

meson compile:

ninja: error: 'build/extensions/Foo-1.0.gir', needed by 'extensions/Foo-1.0.typelib', missing and no known rule to make it

Currenty, documentation for vala is wrong, and you can look around for projects using vala libraries and how they link it

https://github.com/akiraux/libgtkcanvas/blob/master/src/meson.build#L88

I will be glad to improve this documentation once related PR are merged