Fix meson deprecation warnings

[vmihalko]

This PR fixes the following deprecated feature warnings:

• 0.55.0: {'ExternalProgram.path'}
  • https://mesonbuild.com/Reference-manual_returned_external_program.html#external_programpath
• 0.56.0: {'dependency.get_pkgconfig_variable'}
  • https://mesonbuild.com/Reference-manual_returned_dep.html#depget_pkgconfig_variable
• 0.62.0: {'pkgconfig.generate variable for builtin directories'}
  • https://mesonbuild.com/Release-notes-for-0-62-0.html#pkgconfiggenerate-will-now-include-variables-for-builtin-directories-when-referenced

And the following warning (related issue https://github.com/mesonbuild/meson/issues/9300):

WARNING: You should add the boolean check kwarg to the run_command call.
         It currently defaults to false,
         but it will default to true in future releases of meson.

[vmihalko]

Thank you for this contribution!