Building with clang 16 + libc++

Downstream bug: https://bugs.gentoo.org/909316

Apparently, libc++ decided to remove the <experimental/filesystem> header, regardless of the chosen c++ standard version. That means that this application now breaks with libc++, even when built with -std=c++14.

These headers were never meant to last, I don't think, and will be slowly phased out. FreeBSD has created a set of patches that change the build from c++14 to c++17, in order to use the header directly, as well as fix a missing include.

Since we don't carry any compilers that don't support c++17 with anymore, we'll be applying these patches downstream as well. I'd recommend to do the same upstream, unless you're trying to target a compiler that doesn't support c++17, but only c++14 with the experimental header. I know telegram-desktop's official builds use an ancient compiler, maybe that's an issue?