So it seems that [[]] are needed to quote the command part of
AC_CONFIG_COMMANDS(), in particular when square brackets are used inside
the command (which is our case since we used sed with regex classes []).
It is not even in the docs of AC_CONFIG_COMMANDS() but I found this
information in the docs of the deprecated AC_OUTPUT_COMMANDS()!
https://www.gnu.org/software/autoconf/manual/autoconf.html#index-AC_005fOUTPUT_005fCOMMANDS-2135

> Conversely, where one level of quoting was enough for literal strings
> with AC_OUTPUT_COMMANDS, you need two with AC_CONFIG_COMMANDS. The
> following lines are equivalent:
>    AC_OUTPUT_COMMANDS([echo "Square brackets: []"])
>    AC_CONFIG_COMMANDS([default], [[echo "Square brackets: []"]])