Define DLLTOOL for cross-compilation abstraction

This can be used to create custom import libraries when necessary.
prep_symlinks: avoid .exe handling bug in coreutils

https://cygwin.com/ml/cygwin/2014-08/msg00282.html
https://cygwin.com/ml/cygwin/2014-09/msg00364.html
README: fix typo
Fix X session shortcuts for changes in run and xinit
make_etc_defaults: ignore keepdir markers
cygautoconf: export DATADIRNAME

This is necessary for packages which use intltool but not glib-gettext.
cygport 0.17.1