Fix the doxygen target in the top-level Makefile.
diff --git a/Makefile b/Makefile
index f65aa60..4f8cc20 100644
--- a/Makefile
+++ b/Makefile
@@ -14,7 +14,10 @@
 
 
 doxygen:
-	(cd doxygen ; make) ; \
+	cd doxygen && $(MAKE)
+
+.PHONY: doxygen
+
 
 clean:
 	@for dir in $(SUBDIRS) ; do \