Unix: Fix Broken Include (& Add Test)

[ax3l]

Add a check that the path is not only where we want it but files also exist.

Added without logic change to current builds (Re: #43).
Thanks to @henryiii for reporting this! ✨

cc @sdebionne you might want to take a look at this (Re: #32)

Checklist

• Used a fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[ax3l]

ping @wjakob @SylvainCorlay: this conda-forge bugfix for Unix builds is ready to be reviewed & merged.

[henryiii]

Master should be checked before the next release to make sure it's still working (after this is merged).

[ax3l]

Yes, this will automatically be checked when it goes through here.

[ax3l]

(Just amended the meta-data of the last commit to put authorship on you, since I did not see initially that you also changed that line in #43. Sorry again for this.)

[ax3l]

@conda-forge-admin, please rerender

[ax3l]

Excellent, now that it's your code I can review and merge it myself :-p
Jokes aside, thanks again :)