Notes on Windows interface.

Two versions of the Windows graphic interface can be used, a pthreads version
and a Winthreads version.  

For a 32-bit compile either may be used.
For a 64-bit compile the pthreads version should be used to avoid problems
with the multi-threading to support the graphics and screen IO (mouse and
keyboard input).  This is now the default version.

Two versions of the pthreads interface are available, one uses pthreads mutex 
for managing shared data between the main thread and the graphics thread that 
manages mouse and keyboard events. The alternate version uses a blocking flag 
to manage access to the shared event data.  

The mutex version is the preferred interface code.

The choice of interface source file is set in the config.make.xxx files in
the baseline plotlib directory.