Release 3.1.
release.sh: make the script NixOS-friendly, fix quoting in echo command.
release.sh: make the script NixOS-friendly, fix quoting in echo command.
Makefile.lib.am: remove nonexistent header from the list.
Update CHANGELOG.
Document configure option --enable-parsers in BUILD.md.
Update documentation for capturing and non-capturing groups.
Add tests for `re2c:leftmost-captures` and `re2c:posix-captures`.