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.

[genivia-inc]

Thank you for contributing. I hope you are sympathetic with my point that it is a burden for me to maintain multiple build systems. I wish you had a way to detect SIMD to accelerate the library, especially for searching (input scanning/lexing isn't sped up much). Can't you just run the build.sh script that does not use a third-party build system? The build.sh script detects SIMD.

[bredelings]

I added the SIMD support to the meson.build by updating the meson script from the "wrapdb" that @dnicolodi mentioned.

If you want me to update the "wrapdb" and leave meson out of this project, that is OK. However, it might be simpler to host this here. We could add the tests to meson.build, and then add a github actions job to make sure that compilation under meson works and the tests run.

It seems to me like updating this file would mostly require changing the list of files if you add, rename, or delete files. What do you think?

[bredelings]

I haven't added any github actions tests, pending the other PR that adds github actions tests for the autotools setup. However, if some version of that is merged, adding a github actions test for meson would make sense.

I could also possible try and update the CMakeLists.txt to build the SIMD libraries, and add a test for that.