Meson: Fix building conntest sample without curl (backport #2964)

[mergify[bot]]

Somewhat related to #2963 and #2961

When building with Meson the dependency logic made it impossible to build the conntest sample without curl, even though webready can be enabled in a limited, http://-only mode that doesn't require curl.

This PR separates the curl_dep dependency for libcurl from the web_dep dependency that indicates whether the webready option is enabled, in the meson configuration. The latter is now used both to determine whether EXV_ENABLE_WEBREADY should be defined, and whether conntest should be built at all.

When web_dep represents a webready-enabled configuration, curl_dep is set as a dependency. Anything depending on web_dep (currently only conntest) will be linked with libcurl iff it's available.

---

This is an automatic backport of pull request #2964 done by Mergify.

[mergify[bot]]

Cherry-pick of ebd4e44004890a30f223afca41607a0742ee5734 has failed:

On branch mergify/bp/0.28.x/pr-2964
Your branch is up to date with 'origin/0.28.x'.

You are currently cherry-picking commit ebd4e4400.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
    both modified:   meson.build

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally