maint: import modern bootstrap

Copy the latest gnulib bootstrap, which runs autoreconf and
generates po/Makevars for us.  Other improvements include some
improved prerequisite tool checking.

This also fixes a bug in the .pot files, regarding the copyright holder.

* bootstrap: Update to version in .gnulib/build-aux.
* bootstrap.conf (MSGID_BUGS_ADDRESS, COPYRIGHT_HOLDER, SKIP_PO)
(gnulib_mk, ACLOCAL, bootstrap_epilogue): Provide overrides.
* autogen.sh (autoreconf): Avoid redundant autoreconf if bootstrap
was run.
* po/Makevars: Delete, now that bootstrap creates it.
* po/.gitignore: Update.

autogen.sh

@@ -67,7 +67,8 @@ fi
 curr_status=.git-module-status
 t=$(git submodule status)
 if test "$t" = "$(cat $curr_status 2>/dev/null)"; then
-    : # good, it's up to date
+    : # good, it's up to date, all we need is autoreconf
+    autoreconf -if
 else
   echo running bootstrap...
   ./bootstrap && echo "$t" > $curr_status || {
@@ -76,11 +77,6 @@ else
   }
 fi
 
-# Automake requires that ChangeLog exist.
-touch ChangeLog
-
-autoreconf -if
-
 cd "$THEDIR"
 
 if test "x$OBJ_DIR" != x; then