Meson: fixes, cleanups, and modernizations (#1024)

This series cleans up and modernizes the Meson code a bit:

 - It adds the license SPDX name to the proper field
- Sets a minimum version, the version is fairly old, but without setting
a version then Meson will never warn about issues, including potentially
important ones like "this happens to work in some cases but wasn't
intended and doesn't work in all cases"
 - uses more modern Meson features to make things easier for consumers
- Fixes the tests using the ensure_utf8 helpers by ensuring that they're
actually built, and placing them in the correct path.
 - Adds the Meson test to the CI

.github/workflows/tests.yml

@@ -166,6 +166,9 @@ jobs:
       - name: Build
         run: meson compile -C build-meson
 
+      - name: Test
+        run: meson test -C build-meson
+
   install:
     name: install tests
     runs-on: ubuntu-latest