Allows us to use the PKG_CONFIG variable to tell Meson which is the
pkg-config to be used to get our native dependencies.
We actually could always do that but Meson would cache the variable for
the pkg-config file if we used the dependency() call without 'native:
true' and after that all future usages of dependency() even with
'native: true' would use the cross-compile one.
See this issue from where the patch came from:
- https://github.com/mesonbuild/meson/issues/1736
d642a5493d