pkg-config: Support for arbitrary variables

[fooishbar]

pkg-config supports querying arbitrary variables, which is something we make use of in two places in Wayland.

Firstly, we install a .pc for the wayland-scanner generator/utility, so it's a little bit easier to find and a little bit harder to get wrong. This involves a custom wayland_scanner variable in the pkg-config file.

Secondly, we also use it in other places for the plugin directory setting. GStreamer also does this (and a couple of other things, e.g. GST_API_VERSION), and as a result has to use configure_file rather than built-in pkg-config support. I take it from #620 that this is desirable to fix.

[QuLogic]

This is a dupe of #726 and I think it's available already.

[nirbheek]

Yes, this is already available, however you need 0.36.0 for this:

https://github.com/mesonbuild/meson/wiki/Reference-manual#dependency-object

A workaround if you need to use an older Meson is to use run_command to call pkg-config --variable foo manually.

[fooishbar]

What about arbitrary variables in file generation?

[nirbheek]

We're planning on adding more features to that, but it's not urgent since there's a good workaround. The recommended way to do non-trivial pkg-config file generation is to use configure_file on a .pc.in file. Most projects already have them, and using them helps keep Autotools and Meson build systems in-sync for projects that have both.

I see that in Wayland you also already have .pc.in files, so just running configure_file with install_dir: should work out for you.

[nirbheek]

Closing this since improving pkg-config file generation is already on the TODO list.

[whot]

fwiw, support for variables in file generation was added with commit 7ed1990