use pkg-config to properly link to xkbcommon library when build without dlopen feature

[semarie]

some OS (like OpenBSD or NetBSD) places third-parties libraries in no default paths, and requires explicit configuration to link to them. It makes smithay-client-toolkit difficult to build on such systems, as when not using dlopen feature, the build will fail.

For example, with alacritty build:

Building [=======================> ] 188/189: alacritty(bin)

error: linking with `cc` failed: exit status: 1
  |
  = note: [annoying stuff]
  = note: ld: error: unable to find library -lxkbcommon
          cc: error: linker command failed with exit code 1 (use -v to see invocation)

error: aborting due to previous error

error: could not compile `alacritty`

To learn more, run the command again with --verbose.

Currently, there is no way to specify a directory in smithay-client-toolkit to found the library and properly link to it.

This PR uses pkg-config crate to configure build script to properly locate the library based on pkg-config configuration file.

[semarie]

for reference, the problem was originally reported to alacritty/alacritty#5422

[elinorbgr]

Thanks a lot, this is a clear oversight of when we added support for directly linking to xkbcommon rather than always dlopen-ing it.

I just have a small issue with the build-script logic, and could you also add an entry in the changelog, under the ## Unreleased section and as a ### Bugfixes subection (following the previous entries structure) ?

[semarie]

feel free to told me if you present I push amended commit instead of adding new commits on top of previous one.

[elinorbgr]

Okay, this is good for me, thanks!

Regarding the commits, I tend to try maintaining a succinct git history, so if you can amend your PR to make it as a single commit that would be appreciated. Otherwise I'll just squash-merge it.