[Xlibs] Shared library support on Cygwin

[Harold L Hunt II]

Currently there are two issues with building shared libraries from the 
xlibs repository on Cygwin:

1) We need to set "LDFLAGS = -no-undefined" for all libraries that do 
not export undefined symbols.  There are ways of doing this such that 
this flag is only added when compiling on Cygwin.  However, I do not 
believe that there is an adverse affect to having the flag present when 
not compiling on Cygwin (it should be ignored if shared libs are allowed 
to have undefined symbols on the current platform).  Are there any 
objections to adding this flag to all libs in the xlibs repository that 
do not export undefined symbols?  Also, is there a volunteer that would 
like to do this for me?

2) This may be related to #1... but Xau is being directly linked when 
X11 is linked.  Currently, Xau is built as static (but the question is, 
does it have to be static, or does it not export undefined symbols?); on 
Cygwin, shared libraries cannot be linked to static libraries, so X11 
will be forced to be static only if linked to a static Xau.  I checked 
my build logs for the xc-style tree and I do *not* see Xau being linked 
to X11.  Is there a reason why Xau is being linked to X11 in the xlibs 
tree?  If not, can we remove the -lXau from the X11 link line?

Thanks for any input on these issues,

Harold