Add NFD_INSTALL option + disable install target for subproject usage

[encounter]

This checks whether the project is being included as a subproject, and sets NFD_INSTALL and NFD_BUILD_TESTS accordingly.

Note: this does change the default for NFD_BUILD_TESTS to ON when being built as the root project, this can be reverted if not desired.

[btzy]

Thanks for the PR! This seems reasonable. Tests were not built by default since #17, which seems to be using NFD in another CMake project.

Also, I'm wondering if this is the "correct" solution to this problem in CMake - it seems like a fairly common use case. The only resource on the Internet that I could find describing this solution is here. Is this what large CMake projects usually do?

[encounter]

As far as I’m aware, this is the proper CMake solution. It’s in all the large CMake projects I’ve seen. (Though I agree it appears slightly janky 🙂)

SDL2 example: https://github.com/libsdl-org/SDL/blob/main/CMakeLists.txt#L8

[btzy]

I see. I did a check on the list of popular projects known to use NFDe, and it seems that none of them will be affected by the NFD_BUILD_TESTS change.