Support passing pkg-config a triple-specific env variables

[lu-zero]

It should mimic cargo use of triples for the compiler env.

[sdroege]

Do you have some examples how this would be used in practice?

[lu-zero]

in rav1e we tripped in a situation in which an user sets PKG_CONFIG_PATH to a cross-build path and somehow it trips since our build.rs that requires to link host stuff.

In would be good to have a mean to have different search paths per target to avoid the or even just have a HOST_/TARGET_ split.

[nirbheek]

Ideally you would invoke pkg-config yourself with the right env vars, instead of having pkg-config understand build / host etc. This also allows you to extend the same to other dependency search tools like cmake and qmake. This is exactly what meson does: --build.pkg-config-path --build.cmake-prefix-path vs --pkg-config-path --cmake-prefix-path.

[gdesmott]

even just have a HOST_/TARGET_ split.

Looks like pkg-config-rs, used by system-deps already support HOST/TARGET env variables. Would this do or what are you looking for exactly?

[lu-zero]

Probably it does, I opened the issue to forward the concern a rav1e user had.