[nfc][cmake] mention that some builtins are no longer bundled

[ferdymercury]

but require internet connection

[github-actions]

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[dpiparo]

Thanks. When zstd is merged #21597 and zlib is added to the bundle, this will be merged (and maybe topped up with lzma, which is treated the same.
This nice doc will be complemented with something more substantial in the Release Notes and in the repository as well, to give a clear and simple overview of what builtins are used and what is their version.

[ferdymercury]

Thanks for the feedback Danilo :)

btw, builtin_xxhash looks like another candidate for off-loading to LCG, in the context of compression

[silverweed]

btw, builtin_xxhash looks like another candidate for off-loading to LCG, in the context of compression

AFAIK the policy is to keep anything that is strictly required to build minimal ROOT as vendored code and xxhash is one such dependency (required e.g. to build RNTuple)

[hageboeck]

btw, builtin_xxhash looks like another candidate for off-loading to LCG, in the context of compression

AFAIK the policy is to keep anything that is strictly required to build minimal ROOT as vendored code and xxhash is one such dependency (required e.g. to build RNTuple)

I think we are about to abandon this policy. For things that are readily available in most Linuxes and in homebrew, we suggest, in order of preference:

1. Get it from the system
2. Compile and install it to a custom location and make CMake aware of it (e.g. using CMAKE_PREFIX_PATH)
3. Ask ROOT to download the builtin (requires network)
4. [Not implemented yet] Provide a folder that contains the required tarballs. We are considering a config option that would tell ROOT where to pick up the tarballs.

Only option 3 requires network during the configure step. All others could be performed offline.

[ferdymercury]

Suggested change

	ROOT_BUILD_OPTION(builtin_xxhash OFF "Build bundled copy of xxHash")
	ROOT_BUILD_OPTION(builtin_xxhash OFF "Build xxHash internally (requires network)")

To apply if #21806 gets merged

[ferdymercury]

Suggested change

	ROOT_BUILD_OPTION(builtin_pcre OFF "Build bundled copy of PCRE")
	ROOT_BUILD_OPTION(builtin_pcre OFF "Build PCRE2 internally (requires network)")

If #21857 gets merged