Fix clang format target command

[nhanders]

closes #21

Summary

When using multiple sources/headers, the building of the custom target clang-format was failing.

This was fixed by using the WORKING_DIRECTORY argument of add_custom_target.

Side Notes

I know you said you'd fixed this in the next release @filipdutescu, but I thought I'd help you along here.

Thank you for putting this template together! The world of modern C++ can be extremely confusing, but having a template like this with best practices has already made a world of difference to me!

Testing

✅ Able to reproduce error by adding src/main.cpp and building as an executable with ${PROJECT_NAME}_BUILD_EXECUTABLE set to ON. This work fixed this error. ✅ Able to run clang-format target with the setup described in the test above

Pull Request Readiness Checklist

See details at CONTRIBUTING.md.

• [x] I agree to contribute to the project under modern-cpp-template (Unlicense) License.
• [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with modern-cpp-template
• [x] The PR is proposed to proper branch
• [x] There is reference to original bug report and related work
• [x] There is accuracy test, performance test and test data in the repository, if applicable
• [x] The feature is well documented and sample code can be built with the project CMake

[filipdutescu]

hey! thanks very much for your kind contribution! please accept my apologies for taking so long before acknowledging it. I will review the PR today, and leave comments afterwards, but I just wanted to thank you and apologize, before anything else

[filipdutescu]

looks great and such an elegant solution! thanks a lot!