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

[mergify]

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.

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

[mergify]

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