v2.3.1
Add tests/IdnaTest.*
Move calls that rely on iconv to test-locale.c, and skip it when no iconv. Fixes: #84.
Update copyright years. Add custom my-update-copyright rule.
valgrind: use suppressions file by default
Add test-glibc, based on API traces from the glibc AI_IDN tests
Add compiler-suggested function attributes
Build fixes for newer gnulib.