Improve C# support on Cygwin.

Improve C# support on Cygwin.

* autogen.sh (GNULIB_MODULES_TOOLS_FOR_SRC): Add cygpath.
* gettext-tools/src/read-csharp.c: Include cygpath.h.
(msgdomain_read_csharp): Convert the directory passed to msgunfmt.net.exe from
Cygwin to Windows syntax.
* gettext-tools/src/read-resources.c: Include cygpath.h.
(read_resources_file): Convert the file name passed to msgunfmt.net.exe from
Cygwin to Windows syntax.
* gettext-tools/src/write-resources.c: Include cygpath.h.
(msgdomain_write_csharp_resources): Convert the file name passed to
msgfmt.net.exe from Cygwin to Windows syntax.