Respect the PKG_CONFIG environment variable when building

[chewi]

This variable is commonly used by other build systems and using it can help with cross-compiling. Falls back to pkg-config as before.

[DerDakon]

Just to clarify: this has been used in Gentoo for a while.

[aclark4life]

@chewi @DerDakon LGTM, thanks. Any risk to approving this for tomorrow's release? I assume not, but if there is a risk, then we'll probably want to wait for 6.2.0… we definitely don't want to make a setup.py change and then have to release a 6.1.1 in a week 😄

[chewi]

It's pretty straightforward, it falls back to the old behaviour, we've had it in Gentoo a while, and your CI has (almost) passed. I think we're good. 😄

[aclark4life]

@hugovk @radarhere OK if I merge this?

[hugovk]

Fine by me.

Is there some documentation on the PKG_CONFIG environment variable? From a quick search I could only find PKG_CONFIG_PATH.

https://linux.die.net/man/1/pkg-config

[DerDakon]

The latter is how to control pkg-config, i.e. where it looks for it's stuff. The former is how a build system picks up which pkg-config or replacement executable, which is a feature of the build systems. The Linux kernel does it: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/build/feature/Makefile#n72 Others are probably doing the same. CMake has a different variable for the same purpose, but also looks ok PKG_CONFIG as a hint: https://cmake.org/cmake/help/latest/module/FindPkgConfig.html#variables-affecting-behavior