Fixing math example on Linux

[BradLarson]

The usage of sqrt() requires linking libm on Linux. This adds the appropriate CMake entry to allow for this.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test

[natecook1000]

@swift-ci Please test

[natecook1000]

@BradLarson Thanks for these fixes! I’ll work on getting a CMake build added as part of the CI.

[BradLarson]

The one to thank for these is actually @compnerd. He walked me through the fixes as part of related work on a separate PR that I was encountering CMake issues with.

We've been using ArgumentParser in other existing targets there, but I realized none were set up for CMake yet. This was the first.