This was removed on 2014-10-29[1], but the docs weren't updated.
- cross packages use the prefixed ${target}-pkg-config wrapper
- native packages use usr/${build}/bin/pkgconf
- native reqs may install whichever pkg-config the package manager
specifies, but it isn't used by mxe
closes#2456
[1] a063f823e8 (diff-b67911656ef5d18c4ae36cb6741b7965)