[Xlibs] Shared library support on Cygwin

[Keith Packard]

Around 23 o'clock on Jan 13, Harold L Hunt II wrote:

> `-no-undefined'
>      Modern architectures allow us to create shared libraries with 
> undefined symbols, provided those symbols are resolved (usually by the 
> executable which loads the library) at runtime.

The problem with Xlib is that it has undefined symbols which *aren't*
resolved at run-time unless you happen to link your application with
-lpthread (which only pthread-based multithreaded applications do).  I
think this flag will cause problems in such environments, but I admit that
I don't really know.

-keith