Set conditional HTTP depending on EXIV2_ENABLE_WEBREADY (backport #2844)

[mergify]

HTTP support is being enabled regardless of the fact EXIV2_ENABLE_WEBREADY is configured or not.
This PR makes sure http is not included/built if EXIV2_ENABLE_WEBREADY specifically disabled.

This is an alternative to #2843
PoC: xbmc/xbmc#24109

---

This is an automatic backport of pull request #2844 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 9f90144 has failed:

On branch mergify/bp/0.28.x/pr-2844
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Changes to be committed:
	modified:   include/exiv2/exiv2.hpp
	modified:   include/meson.build
	modified:   meson.build
	modified:   meson_options.txt
	modified:   src/basicio.cpp
	modified:   src/image.cpp

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/CMakeLists.txt

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