Make: Fix configure generated for dist-client

Commit 1ec9b94 broke regeneration of configure for dist-client: it
uses .tarball-version, which exists only in the tarball, not in the
source tree.

Generating the client's autoconf stuff into the source tree is a
questionable idea anyway, as it won't run there.  Generate it while
building the tarball instead.

Bonus: doesn't distribute a useless .dirty-stamp.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>

src/scripts/gen-client-configure

@@ -0,0 +1,13 @@
+#!/bin/sh -e
+
+if [ $# = 0 ]; then
+    echo "Usage: $0 NAME VERSION (FILE | -C DIR)..."
+    exit 1
+fi
+
+cd $1-$2
+printf "%s" $2 >.tarball-version
+cat m4/*.m4 >aclocal.m4
+autoconf
+autoheader
+rm -rf m4 autom4te.cache