Fix for aclocal-m4-deps.mk build error in incremental builds

[Christian Biesinger via gdb]

Hi all,

if you see this build error:
make[2]: *** No rule to make target '../../gnulib/aclocal-m4-deps.mk'.  Stop.

You can fix it by going into the gnulib directory in your build dir
and running ./config.status.

I broke this in commit 33bd0102c190c1331804c12774e578e33c367552 which
removed the file, because it will no longer be needed, but I missed
the fact that this would break incremental builds.

Christian