Add dummy_thread[ing] sections and reference these from libthread[ing].

Doc/lib/lib.tex

@@ -164,6 +164,8 @@ and how to embed it in other applications.
 \input{libselect}
 \input{libthread}
 \input{libthreading}
+\input{libdummythread}
+\input{libdummythreading}
 \input{libqueue}
 \input{libmmap}
 \input{libanydbm}

Doc/lib/libthread.tex

@@ -17,7 +17,9 @@ semaphores}) are provided.
 
 The module is optional.  It is supported on Windows, Linux, SGI
 IRIX, Solaris 2.x, as well as on systems that have a \POSIX{} thread
-(a.k.a. ``pthread'') implementation.
+(a.k.a. ``pthread'') implementation.  For systems lacking the \module{thread}
+module, the \refmodule{dummy_thread} module is available.  It duplicates this
+module's interface and can be used as a drop-in replacement.
 \index{pthreads}
 \indexii{threads}{\POSIX}
 

Doc/lib/libthreading.tex

@@ -8,6 +8,9 @@
 This module constructs higher-level threading interfaces on top of the 
 lower level \refmodule{thread} module.
 
+The \refmodule{dummy_threading} module is provided for situations where
+\module{threading} cannot be used because \module{thread} is missing.
+
 This module is safe for use with \samp{from threading import *}.  It
 defines the following functions and objects: