Add Meson build

[bredelings]

I'm adding reflex as a subproject in a C++ project that is managed with meson.

I can include a patch in the meson "wrap" that adds simple meson.build file to reflex, but it would be much nicer if this already existed upstream.

[dnicolodi]

I was maintaining a meson build definition for RE/Flex in the WrapDB https://github.com/mesonbuild/wrapdb/blob/master/subprojects/packagefiles/reflex/ but I stopped updating it a while ago. See also #219.

It includes SIMD detection and it is tested on a few platforms. However, I stopped updating it a while ago, at version 5.1.1 (no time for the side project where it was used). Maybe you can open a PR to update that.

I think that, unless the RE/Flex maintainer commits to keep the in-tree build definition current with future updates to the codebase, it is easier to maintain it outside the project. WrapDB makes it trivial to add it as a subproject to projects that need it.