Missing gtkdoc-mktmpl
Trying to build with current gtk-doc fails with:
gtkdoc-mktmpl --module=libidn2
make[4]: gtkdoc-mktmpl: Command not found
make[4]: *** [Makefile:1260: tmpl-build.stamp] Error 127
It appears that gtk-doc 1.26 removed the gtkdoc-mktmpl command.
If you update to a newer gtkdoc, then rerun gtkdocize, automake and autoconf then it should generate a build process that has no reliance on the now missing gtkdoc-mktmpl command.