Support object library

[0x1997]

I'm trying to embed a few dependencies into a shared library. It seems impossible to set compiler flags on a group of source files gathered by files(). But building static libraries of the dependencies with -fPIC and bundling them into the shared library with -Wl,--whole-archive or extract_all_objects seems inconvenient and a bit hacky.
I'd like to have an object library which is just a bunch of object files that I can set compiler flags as a group and add to any real executable or library target.

[elmarco]

Another approach that could work is to allow per-files() args and dependencies (this would avoid having to name a library or giving a namespace for unit objects, as the files() are associated with an existing target). files('foo.c', dependencies: dep). I have a proof-of-concept branch on https://github.com/elmarco/meson/tree/files-deps

[bonzini]

I think this can be closed since support for extract_objects has improved a lot and declare_dependency(objects: ...) makes it possible to treat "object libraries" the same as static libraries. There are some open issues (#13843 for documentation and #14031 for Rust for example), but overall the functionality is there.