Add pkg-config to build-essential

[RantyDave]

I was just attempting to build a project that uses autogen and was failing in a non-obvious manner (error: possibly undefined macro). The fix was to install pkg-config and adding this to build-essential would have prevented the problem occurring in the first place.

[jperkin]

Thanks for the patch. Unfortunately it's not quite that simple. Since last April we switched to devel/pkgconf by default (BSD licensed compatible replacement), which conflicts with the original pkg-config. So what needs to happen here is devel/pkg-config pulled in for earlier branches (up to 2016Q1 I presume but would need to double check) and then devel/pkgconf for all branches since then.

This is needed as pkgsrc-joyent is currently shared between branches, though at some point we should probably switch to giving it branches of its own to avoid this requirement.

I'll try and get this done in the next day or so. Thanks!