Add libwslay.pc pkgconfig file

It would be great if you could generate and add a libwslay.pc pkgconfig file matching the distribution.

wslay's autotools buildsystem used to generate one from https://salsa.debian.org/debian/wslay/blob/master/lib/libwslay.pc.in, but you're now using the CMake buildsystem, which ignores it.

For my Mageia RPM package I created it manually, as I didn't want to bother fixing CMake for that, but the latter is of course an option. There's actually an unmerged PR upstream for that, though I don't know how well it would play with Debian's fixes to the library name and SONAME.

Finally, the motivation: we're adding wslay as a dependency in Godot Engine (replacing libwebsockets), which is packaged in Debian as godot3. The Debian package still tracks Godot 3.0.x, while the wslay dependency will be added in 3.2. When unbundling the libraries we vendor, we query pkgconfig from SCons, so it would be great if Debian (and Ubuntu) had a libwslay.pc file.