xorg/system: Support PkgConfigDeps

[jwillikers]

Fix file names of *.pc files when using PkgConfigDeps generator.
Include custom variables in *.pc files.

Fixes a long-standing issue of a missing variable when building xkbcommon.
Fixes #3377.

Specify library name and version: xorg/system

---

• I've read the guidelines for contributing.
• I've followed the PEP8 style guides for Python code in the recipes.
• I've used the latest Conan client version.
• I've tried at least one configuration locally with the conan-center hook activated.

[conan-center-bot]

All green in build 3 (1ff1c3a67fcac72da57943d679c9694b89c0334f):

• xorg/system@:
  All packages built successfully! (All logs)

  🔸 Informative: This recipe is not ready for Conan v2

  _{We have started the migration process to Conan v2 and exporting recipes successfully will be required in the future.
  This is just an informative note to gain awareness about the process, no need to take any action. The plan is to enforce smaller steps that are easier to fix and, eventually, this conan export step will work.
  See the recipe migration guide to know more about the changes required.}

  ERROR: Error loading conanfile at '/home/conan/w/prod_cci_PR-11634/recipes/xorg/all/conanfile.py': Unable to load conanfile in /home/conan/w/prod_cci_PR-11634/recipes/xorg/all/conanfile.py
    File "<frozen importlib._bootstrap_external>", line 728, in exec_module
    File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
    File "/home/conan/w/prod_cci_PR-11634/recipes/xorg/all/conanfile.py", line 1, in <module>
      from conans import ConanFile, tools
  ImportError: cannot import name 'ConanFile' from 'conans' (/opt/pyenv/versions/3.7.13/lib/python3.7/site-packages/conans/__init__.py)