Fix generation of README.rst, to get make distcheck to succeed.

Makefile.am

@@ -9,12 +9,10 @@ EXTRA_DIST = README.rst LICENSE COPYING CONTRIBUTING.rst INSTALL.rst
 
 doc_DATA = README.rst LICENSE COPYING CONTRIBUTING.rst INSTALL.rst
 
-README.rst: src/vmod_selector.man.rst
+README.rst: src/vmod_selector.vcc
+	$(MAKE) $(AM_MAKEFLAGS) -C src vmod_selector.man.rst
 	cp src/vmod_selector.man.rst README.rst
 
-src/vmod_selector.man.rst:
-	make -C src vmod_selector.man.rst
-
 %.1 %.2 %.3 %.4 %.5 %.6 %.7 %.8 %.9:
 if HAVE_RST2MAN
 	${RST2MAN} $< $@