this library is bad, and its documentation is even worse

and our linenoise now supports every non-boomer platform already
anyway

apparently macos c++ stdlib is missing c++20 features (particularly
string_view doesn't seem to accept two pointers in a constructor)
and mingw is only present as gcc9 in ubuntu 20.04 (too old)

this brings more advantages too like actually being default
constructible, so it can be used more easily in data structures etc

later commits will rewrite the parser and separate the codegen
bits into their own API

since meson will always default to pic for static libraries unless
overridden with -Db_staticpic=false, we don't want to mess with
this

since meson doesn't yet support cpp_std=c++2a or c++20 for msvc++,
only gcc (and the older version we support doesn't have c++20
either), default to none and add some minimal logic which will
guess it for whatever compiler we're using

people can still override the standard manually, in which case our
guess logic will not be used

users of the library never need to know this