LGTM

Are you sure that going against the normal behavior of pkg-config is a good solution to whatever problem you were having?

Sorry for the rather terse PRs. It's not normal behaviour. We're working around a bug in pkg-config which AFAICT was exposed by the use of multiple directories in PKG_CONFIG_PATH and resulted in the link order for Solo5 unikernels being wrong (libmirage-solo5_bindings.a was placed after the ocaml-freestanding asmrun.a).

Best as I can tell(*) the bug only affects pkg-config versions prior to 0.28 from January 2013, unfortunately Ubuntu 14.04 (used by Travis) ships 0.26(!). @hannesm has also verified that this does not break on FreeBSD which uses it's own pkg-conf reimplementation of pkg-config.

(*) I think this is the commit that fixes the pkg-config bug: https://cgit.freedesktop.org/pkg-config/commit/?id=6ecf318c927ffe89a028b8b9cb70d0535673c550

<rant>
Our use of pkg-config is a house of cards in any case, since we depend on the system pkg-config and then manipulate it's search path via PKG_CONFIG_PATH.

A possible improvement might be to ship our own build of pkg-config (or the FreeBSD implementation thereof) as part of conf-pkg-config with the correct paths already set and call it something like opam-pkg-config. This could then be replaced with a thin wrapper over per-package variables for OPAM 2.0.
</rant>

/cc FYI @avsm @yomimono @talex5

Thanks for this explanation @mato. I would suggest to use it as is for now, and make a switch over to opam 2.0 once that is available and stable. No need to work on improving the pkg-config situation in the meantime (as it is right now it seems to work on the platforms we're interested in).

I agree, unless it breaks again in some unforeseen way, in which case we'll need to fix it properly.