feat: use pkg-config to discover pcsclite

[rawkode]

This was already mentioned in the comments, but not implemented. This allows for piv-go to be used successfully as a dependency on Nix builds; as well as being generally more predictable.

[ericchiang]

To echo @FiloSottile, please only make changes to platforms you're able to test this on. This doesn't build on MacOS :)

% go test -v .
# pkg-config --cflags  -- libpcsclite
pkg-config: exec: "pkg-config": executable file not found in $PATH
FAIL    github.com/go-piv/piv-go/piv [build failed]
FAIL

Is it possible to specify pkg-config and fallback to CFLAGS/LDFLAGS if that's not available?

[flokli]

Yeah, it seems pkg-config and the appropriate .pc files aren't available and properly installed on all platforms.

According to https://github.com/golang/go/blob/master/src/go/internal/srcimporter/srcimporter.go#L219-L232, these directives should be additive, and all their outputs are passed to the compiler - So it shouldn't hurt to just add the pkg-config line as well with the others.

#cgo linux {C,LD}FLAGS could be dropped, if we assume at least all linux distros properly have pkg-config set up.

[flokli]

@rawkode can you update the PR?

[rawkode]

As per feedback, I've updated the PR so that Linux uses pkg-config and MacOS and FreeBSD are untouched.

[philandstuff]

I’m not sure this works on macOS yet. See NixOS/nixpkgs#93769 (comment) Is there a way to ensure pkg-config is only tried on Linux?

[ericchiang]

lgtm. mind squashing then I'll merge? :)

[rawkode]

@ericchiang Sorted

Thanks for everyone's help, @flokli @philandstuff @FiloSottile