Fix tangling

src/Makefile.am

@@ -104,7 +104,10 @@ DISTCHECK_CONFIGURE_FLAGS = \
 include $(top_srcdir)/am/global-rules
 
 BUILT_SOURCES = timestepping.hh timestepping.cc
-$(srcdir)/timestepping.cc $(srcdir)/timestepping.hh: $(srcdir)/timestepping.org
+# Make sure the two are not built in parallel
+$(srcdir)/timestepping.cc: $(srcdir)/timestepping.hh
+
+$(srcdir)/timestepping.hh: $(srcdir)/timestepping.org
 	emacs -Q --batch --eval \
 	  "(let (vc-handled-backends) \
 	      (org-babel-tangle-file \"$<\" nil 'c++))"