Add a docs target to make

Now make has a docs target to enforce docs generation

Makefile.am

@@ -3,4 +3,7 @@ CLEANFILES =
 #ACLOCAL_AMFLAGS = -I m4
 if HAVE_DOXYGEN
 SUBDIRS += docs
+docs:
+	-cd docs; rm doxyfile.stamp; make doxyfile.stamp
+.PHONY: docs
 endif