Add std/pkgconfig module

[ehmry]

A module for collecting compile-time information from pkg-config.

I've been using pkg-config with passC for years on Linux and it's worked well.

Using $PKG_CONFIG with a fallback to pkg-config is useful for cross-compiling.

[planetis-m]

To be fair this is something that could belong in fusion...

[ghost]

If it was something that I would have to fetch an additional package for then I would continue using it as boilerplate to paste into modules.

[arnetheduck]

Indeed, or a nimble package that could follow a separate release cycle and prove is utility there..

[ghost]

I'm still copying this module from project to project and I haven't made any changes to it.

The motivation for having this in std is to encourage people to use pkg-config instead of hardcoding some works-on-my-machine compiler and linker flags. It if was a Nimble package then no one would use it because it takes more time to add another dependency to a project than it does to copypaste whatever compiler flags happen to be working at the moment.

[Araq]

Fair enough but can't this assume e.g. "Scoop" on Windows and improve on the status quo (people not caring about Windows...)