Currently, this library picks up

<build-kind>_<var> - for example, HOST_PKG_CONFIG_PATH or TARGET_PKG_CONFIG_PATH

This is fine, but this is quite non-standard. A standard way of doing things is instead using a _FOR_BUILD suffix:

• TARGET_PKG_CONFIG_PATH = PKG_CONFIG_PATH
• PKG_CONFIG_PATH = PKG_CONFIG_PATH_FOR_BUILD

This is what we do in Nipxkgs, and also what Meson does (see https://mesonbuild.com/Reference-tables.html#environment-variables-per-machine). This convention goes back to autotools.

CC @dcbaker @alyssais @lovesegfault