add check for librapidcheck

[NaN-git]

Motivation

So far configure.ac does not check whether librapidcheck can be linked. Also there is no explanation of RAPIDCHECK_HEADERS, which makes proper builds without nix difficult.

Changes

• add check for librapidcheck
• declare RAPIDCHECK_HEADERS as variable with help string

Context

The missing library check was discussed in #7809.

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• documentation in the internal API docs
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

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[Ericson2314]

I wonder if it might be better to instead allow disabling the unit tests without disabling the functional tests.

What do you think, @robert?

[cole-h]

(Probably meant to ping @roberth?)

[roberth]

I think improving the granularity is probably good, but I don't think this PR's change affects the need for rebuilds in any significant way, so there's no hurry.

[Ericson2314]

Thanks for doing this! Just the one question above from me.

It may make sense to just merge this and return to that after too.

[lf-]

n.b. I was having configure failures with this check, which were baffling. I traced it down to zsh not expanding $configureFlags to multiple arguments, the fix of which is writing ${=configureFlags} for zsh only. I think I will file a PR adding a note of this to the docs.