startxdmcp.bat doesn't discover its path

[James Hanley]

Inside of the ./bin/startxdmcp.bat arround line 30, the script hard codes
the variable assignment as such:
SET CYGWIN_ROOT=\cygwin
This is a non-issue on 32bit installations of Cygwin, but 64 bit seem to
install the environment into C:\cygwin64 and fails to properly run.
Discovering the prefix or pulling it from another external configuration
would allow the script to run properly.
There may be other examples of this in other scripts/batch-files.
-Jim