Use gnulib from a submodule

This allows us to record the exact changeset from gnulib that everybody
should be using.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "gnulib"]
+	path = .gnulib
+	url = git://git.sv.gnu.org/gnulib.git

bootstrap

@@ -33,25 +33,31 @@ do
   esac
 done
 
-cleanup_gnulib() {
-  st=$?
-  rm -fr .gnulib
-  exit $st
-}
+# Get gnulib files.
 
 case ${GNULIB_SRCDIR--} in
 -)
-  if [ ! -d .gnulib ]; then
+  echo "$0: getting gnulib files..."
+  git submodule init || exit $?
+  git submodule update || exit $?
+  GNULIB_SRCDIR=.gnulib
+  ;;
+*)
+  # Redirect the gnulib submodule to the directory on the command line
+  # if possible.
+  if test -d "$GNULIB_SRCDIR"/.git && \
+	git config --file .gitmodules submodule.gnulib.url >/dev/null; then
+    git submodule init
+    GNULIB_SRCDIR=`cd $GNULIB_SRCDIR && pwd`
+    git config --replace-all submodule.gnulib.url $GNULIB_SRCDIR
     echo "$0: getting gnulib files..."
-
-    trap cleanup_gnulib 1 2 13 15
-
-    git clone --depth 1 git://git.sv.gnu.org/gnulib .gnulib ||
-      cleanup_gnulib
-
-    trap - 1 2 13 15
+    git submodule update || exit $?
+    GNULIB_SRCDIR=.gnulib
+  else
+    echo >&2 "$0: invalid gnulib srcdir: $GNULIB_SRCDIR"
+    exit 1
   fi
-  GNULIB_SRCDIR=.gnulib
+  ;;
 esac
 
 gnulib_tool=$GNULIB_SRCDIR/gnulib-tool