[gdome]fix for building on windows

[Tobias Peters <t-peters gmx de>]

File libgdome/gdomecore/gdome-xml-domimpl.c contains a line
extern int xmlDoValidityCheckingDefaultValue;

Could you please remove this line? It prevents building of libgdome on 
windows when libxml is linked in as a dll.

Otherwise it seems that I successfully built libxml and libgdome as 
windows dlls with cygwin. I'll post a summary of how to do it when I can 
repeat the build.

Bye, Tobias