meson: Get rid of cmake and manual library searching for abseil

Simplify fallback, and prefer it. [provide] section needs meson 0.55, so require that.

pkg-config lookup is only provided for distros, since they dislike static linking / vendoring. This is tested on Fedora 37.

Also update abseil-cpp wrap to the latest version and add a tiny change to auto-detect ios as the host_system.