Add meson.build#36

This meson.build file allows you to create a pkg-config and munit library when compiled directly, and include munit as a subproject from external meson projects using subproject() capabilities. See https://github.com/eintw1ck/cha2slide/blob/master/src/meson.build#L75 for an example of how it's used (look for pkg-config version, then vendor if not found by pkg-config). Obviously for munit installing it as a library doesn't make much sense but being able to just set the url to this git repo is very useful (can only be done if the project has a meson.build).