doc: install example .cfg files to $(docdir)/examples/

Change-Id: I8671ce33b9bf28c89f767dd1b4a1463aeb275158
diff --git a/configure.ac b/configure.ac
index 1db32d4..d72a026 100644
--- a/configure.ac
+++ b/configure.ac
@@ -64,6 +64,7 @@
 
 AC_OUTPUT(
 	Makefile
+	doc/Makefile
 	src/Makefile
 	sql/Makefile
 	tests/Makefile