configure: two levels of quoting needed with AC_CONFIG_COMMANDS.

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: []"]])

configure.ac

@@ -2702,7 +2702,7 @@ m4_define([generate_po_makefile], [
   AC_MSG_NOTICE([Generating POTFILES and Makefile for $1])
 
   AC_CONFIG_COMMANDS([$1/stamp-it],
-  [
+  [[
     if  ! grep "^# INTLTOOL_MAKEFILE$" "$1/Makefile.in" > /dev/null ; then
        as_fn_error $? "$1/Makefile.in.in was not created by intltoolize." "$LINENO" 5
     fi
@@ -2716,7 +2716,7 @@ m4_define([generate_po_makefile], [
         -e "d" -e "}"                               \
         "$1/Makefile.in" >"$1/Makefile" && \
     touch "$1/stamp-it"
-    ])
+  ]])
 ])
 
 generate_po_makefile([po-libgimp])