These autoconf helper macros come from various sources:

- ax_*.m4: autoconf-archive, version 2017.09.28 (copied)
- pkg.m4: pkg-config, version 0.29.2 (run configure, then copy)
- gettext.m4, iconv.m4, intlmacosx.m4, nls.m4: gettext 0.19.8
- ctng_*.m4: obviously, implemented anew
- po.m4: a local dummy stub for gettext's version
