CMakeLists.txt: Allow disabling FetchContent

[roxwize]

The CMake configuration script fetches four dependencies from the Internet by default. This results in errors on my end in the form of protocol "https" not supported.

This patchset simply gates the calls to FetchContent that perform this fetching behind the flags FETCH_NLOHMANN_JSON, FETCH_NANOFLANN, etc. If a dependency's respective FETCH flag is disabled, a traditional call to find_package is used instead. For compatibility and consistency reasons, these flags are enabled by default.

Besides fixing the issue stated previously, this patchset also makes configuration faster for people who already have these dependencies downloaded beforehand. In my case, I have a shell.nix that contains lists of the dependencies to be downloaded verbatim that I can then set up a shell environment from, and since I have all of these dependencies, fetching them is unnecessary. I have attempted configuring and building on this patchset with all of the FETCH_* flags set to off, and OpenSplat is able to compile without any issue.

[pierotofy]

Your CMake binary must be compiled without SSL/curl support?

I'm OK adding a flag to disable FetchContent, can you please consolidate into a single flag rather than 4? 🙏

[roxwize]

Yes, I'll do that.

Your CMake binary must be compiled without SSL/curl support?

NixOS, and ergo the version of CMake provided with Nixpkgs, seems to have SSL support disabled. I tried to work around this but ended up just deciding to patch the CMake script to get the dependencies directly instead. https://discourse.nixos.org/t/dealing-with-cmake-fetchcontent/34768

COLMAP uses FetchContent like this as well, but it puts them behind a default-on flag similarly to what I'm doing here.

[pierotofy]

Looks good to me, thanks @roxwize