[Xlibs] Shared library support on Cygwin

[Harold L Hunt II]

Okay, I propose the following:

1) I will sprinkle -no-undefined throughout about 10 top-level 
Makefile.am's in the xlibs repository.

2) If this causes observed bustage on other platforms (please notify me) 
I will create a method of only passing -no-undefined to libtool on 
specific platforms.

I would really like to save the effort in not passing this on other 
platforms if it turns out to be unharmful.

I suspect that this is okay, but I would like at least one person to 
agree before I start committing my patches.

Harold