Fix race in make zipup

`clean` and `docs` run in parallel, so sometimes stuff gets cleaned that
is required to build docs.

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

makefile

@@ -133,7 +133,11 @@ pre_gen:
 	sed -e 's/[[:blank:]]*$$//' mpi.c > pre_gen/mpi.c
 	rm mpi.c
 
-zipup: clean astyle new_file docs
+zipup:
+	$(MAKE) clean
+	$(MAKE) .zipup
+
+.zipup: astyle new_file docs
 	@# Update the index, so diff-index won't fail in case the pdf has been created.
 	@#   As the pdf creation modifies the tex files, git sometimes detects the
 	@#   modified files, but misses that it's put back to its original version.