Avoid building documentation

--- Makefile.am.orig	Sun Sep 20 20:38:23 2020
+++ Makefile.am	Sun Sep 20 20:38:36 2020
@@ -1,5 +1,5 @@
 # Directories where we can build something.
-SUBDIRS          = doc lib-src src
+SUBDIRS          = lib-src src
 
 # Install the examples into /usr/share/doc/cddlib/examples*
 nobase_doc_DATA = $(srcdir)/examples/* $(srcdir)/examples-ine/* $(srcdir)/examples-ine3d/* $(srcdir)/examples-ext/*