build: improve the script to test for Windows installer languages.

We were only comparing the po list with the language list in the
gimp3264.iss. Nevertheless since we also generate the <code>.setup.isl
files, we should also verify generated files corresponds exactly to the
same list of languages.
This commit does it for meson and autotools build.

This is also how I fixed the meson list (cf. previous commit).

(cherry picked from commit 602add8c
except that I removed the comparison with meson files)