Add environment variable to prevent using the system pkgconf

[virtuald]

Referring to #2:

I think for a general user trying to build a package, this seems like a reasonable thing.

However, as someone who wants to distribute packages that only link to other wheels, I definitely don't want to accidentally link to something on the system -- presumably if the package isn't found, then I forgot to include it in build-system.requires?

To accommodate this, can we add an environment variable that if set will prevent using the system pkgconf?