steps towards making 'make distcheck' work

'make distcheck' checks for out-of-source-tree builds and whether
the distribution tarball includes all source files.  This commit
fixes many problems, except some remaining with src/test still
to-be-fixd.
diff --git a/Makefile.am b/Makefile.am
index af437a6..13cec6c 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1 +1,3 @@
 SUBDIRS = src
+
+EXTRA_DIST = asn1