Otherwise the build is cluttered with

```
/nix/store/fwpn2f7a4iqszyydw7ag61zlnp6xk5d3-glibc-2.30-dev/include/features.h:382:4: warning: #warning _FORTIFY_SOURCE requires compiling with optimization (-O) [-Wcpp]
  382 | #  warning _FORTIFY_SOURCE requires compiling with optimization (-O)
      |    ^~~~~~~
```

when building with `OPTIMIZE=0`

This cuts compilation time by ~49s.

Issue #4045.

This reduces compilation time by 207s.

Issue #4045.

This gets rid of the inclusion of <future> in util.hh, cutting
compilation time by ~20s (CPU time).

Issue #4045.

This reduces compilation time by ~15 seconds (CPU time).

Issue #4045.

libfetchers/github: allow `url` attribute

They're still enabled in regular builds though.

Update lowdown version

Fix #4042

According to https://github.com/kristapsdz/lowdown/commit/8aef9e9290de22a10c14ae138257bc1c7fa8ba1f, we shouldn't need to use a fork anymore so we can switch back to upstream

Fix the nix-daemon Mac OS SSL CA cert

nix-prefetch-url: Add --executable flag

pkgs.fetchurl supports an executable argument, which is especially nice
when downloading a large executable. This patch adds the same option to
nix-prefetch-url.

I have tested this to work on the simple case of prefetching a little
executable:

1. nix-prefetch-url --executable https://my/little/script
2. Paste the hash into a pkgs.fetchurl-based package, script-pkg.nix
3. Delete the output from the store to avoid any misidentified artifacts
4. Realise the package script-pkg.nix
5. Run the executable

I repeated the above while using --name, as well.

I suspect --executable would have no meaningful effect if combined with
--unpack, but I have not tried it.

Since 108debef we allow a
`url`-attribute for the `github`-fetcher to fetch tarballs from
self-hosted `gitlab`/`github` instances.

However it's not used when defining e.g. a flake-input

    foobar = {
        type = "github";
        url = "gitlab.myserver";
        /* ... */
    }

and breaks with an evaluation-error:

    error: --- Error --------------------------------------nix
    unsupported input attribute 'url'
    (use '--show-trace' to show detailed location information)

This patch allows flake-inputs to be fetched from self-hosted instances
as well.

find() returns an iterator so "!attr" doesn't work.

Remove storetype delegate reg store -- contains #3736

Fix garbage collection of CA derivations

Remove corepkgs/config.nix

Fix #4026

This isn't used anywhere except in the configure script of the Perl
bindings. I've changed the latter to use the C++ API's Settings object
at runtime.

Mac OS multi-user installations are currently broken because all requests
made by nix-daemon to the binary cache fail with:

```
unable to download ... Problem with the SSL CA cert (path? access rights?) (77).
```

This change ensures that the nix-daemon knows where to find the SSL CA cert file.

Fixes #2899 and #3261.

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

Doesn't test much, but at least ensures that the command runs properly

Add some necessary casts in the initialisation of the store's config

Instead make a separate header with the template implementation of
`BaseSetting<T>::toJSONObj` that can be included where needed

It is apparently required for using `toJSONObject()`, which we do inside
the header file (because it's in a template).

This was accidentally working when building Nix itself (presumably because
`config.hh` was always included after `nlohman/json.hpp`) but caused a
(pretty dirty) build failure in the perl bindings package.

Work around https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80431 that was
already there in the code but was accidentally removed in the last
commits

So that it can be printed by `nix describe-stores`