Add pkg-config to dependency list if building for Linux on Linux
I got this message when building from source on Ubuntu:
```
It looks like you're compiling on Linux and also targeting Linux. Currently this
requires the `pkg-config` utility to find OpenSSL but unfortunately `pkg-config`
could not be found. If you have OpenSSL installed you can likely fix this by
installing `pkg-config`.
```
I feel like it would be a better experience to show this in the dependencies instead of having to run into this issue.