Use the / path operator instead of join_paths() as well as for other paths where now they're just regular strings. Not sure what's idiomatic.
Probably should be https://mesonbuild.com/Syntax.html#string-path-building but windows seems much less strict about the slash/backslash issue these days.
Personally when using meson I gather filenames locally into variables in the respective directories and let meson take the strain, but @kugel- has wisely avoided putting meson.build files into ctags and scintilla directories which will avoid them being clobbered during updates so some path munging is needed for those.