pkgconf escapes isolated build environment

[virtuald]

Accidentally ran into this. When building a meson-python package using python -m build, I expected it to fail because I hadn't added the dependency for pkgconf or the package containing the library... and it didn't fail, because meson found the pkgconf on PATH (as part of my virtualenv) and resolved the library that way.

Probably shouldn't do that.

Mildly related to #53 in that pkgconf might resolve something that wasn't intended.