validate pkg-config output in tests

[minrk]

build test files with pkg-config output instead of repo Makefiles to ensure it has the right info

and fix missing -I$PREFIX/include/mumps_seq for mumps-seq, discovered in conda-forge/ipopt-feedstock#112

Checklist

• Used a personal 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.

[github-actions]

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/meta.yaml) and found it was in an excellent condition.