fj-mkdeb.py: run distclean before ./configure

This (mostly) restores the behavior from before commit 1fb814e
("Makefile.in: stop running distclean on dist", 2022-05-13) / PR netblue30#5142.
./configure still has to be called before calling ./contrib/fj-mkdeb.py
(to generate Makefile from Makefile.in before calling `make distclean`).

contrib/fj-mkdeb.py

@@ -29,6 +29,12 @@ def run(srcdir, args):
         else:
             escaped_args.append(shlex.quote(a))
 
+    # Remove generated files.
+    if not dry_run:
+        distclean = subprocess.call(['make', 'distclean'])
+        if distclean != 0:
+            return distclean
+
     # Run configure to generate mkdeb.sh.
     first_config = subprocess.call(['./configure', '--prefix=/usr'] + args)
     if first_config != 0: