Add check for "ghc-options: -j"

[lspitzner]

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• If the change is docs-only, [ci skip] is used to avoid triggering the build bots.

Please also shortly describe how you tested your change. Bonus points for added tests!

[quasicomputational]

Good idea. Can I ask for a changelog note and a new test for this behaviour? It looks simple but I know I've written simple and wrong code, and it'd keep the feature from accidentally breaking later.

[23Skidoo]

Rebased on top of current master.

[23Skidoo]

LGTM.

[quasicomputational]

Ah, found a nit: the error message mentions ghc-options but it might actually have been in ghc-shared-options or ghc-prof-options. But the rest of the code follows this pattern so I'll open up a bug as a TODO on fixing that.

In the meantime, I pushed a test and a changelog note for this; once Travis comes back green I'll go ahead and push the big green button.