Add CMake interface library target

[legends2k]

Projects using cglm as a header-only library needn’t build files under
src/. Provide a target which allows them to skip compiling them by

add_subdirectory(external/cglm EXCLUDE_FROM_ALL)
target_link_libraries(MyExe PRIVATE cglm_headers)

[legends2k]

This is my first PR here, so I might be completely off, please educate me if I'm missing something; I'm happy to take comments. I've gone through CONTRIBUTING.md too. Thanks!

Bug #181 would be resolved with this issue.

[recp]

Hi @legends2k

Thanks for creating a PR for that issue 🤗

add_subdirectory(external/cglm EXCLUDE_FROM_ALL)
target_link_libraries(MyExe PRIVATE cglm_headers)

We should include this guide to README and docs (docs/source/build.rst) which will help to users to be aware of this feature.

Do we need to add all headers manually in add_target INTERFACE?

Resources for header-only library (but we provide both compiled and header only or header-only):
[0] http://mariobadr.com/creating-a-header-only-library-with-cmake.html
[1] https://hsf-training.github.io/hsf-training-cmake-webpage/04-targets/index.html

[legends2k]

@recp You're right, I've removed the header listing as it's needless. Thanks for reviewing the change.

Found a related post: Opt-in header-only libraries with CMake; example is equivalent to cglm: shared, static and header-only.

[recp]

@legends2k the PR is merged, thanks for your contributions 🎉