doc: re-enable building the dtinfo guides on netbsd

As long as you have enough space in /tmp (10MB is the default! change
this!) these will build fine.

cde/doc/en_US.UTF-8/Makefile.am

@@ -1,8 +1,3 @@
 MAINTAINERCLEANFILES = Makefile.in
 
-if !NETBSD
 SUBDIRS = m-guides man guides help
-else
-# building the guides on netbsd (9.2) doesn't work currently
-SUBDIRS = m-guides man help
-endif