Meson: Fix building conntest sample without curl

[ferdnyc]

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.

[kmilos]

@mergify backport 0.28.x

[mergify]

backport 0.28.x

✅ Backports have been created

• #2974 Meson: Fix building conntest sample without curl (backport #2964) has been created for branch 0.28.x but encountered conflicts