[ From http://cygwin.com/licensing.html ]

              What are the licensing terms?

  Most of the tools are covered by the GNU GPL, some
  are public domain, and others have a X11 style
  copyright. To cover the GNU GPL requirements, the
  basic rule is if you give out any binaries, you must
  also make the source available. For the full details,
  be sure to read the text of the GNU General Public
  License (GPL).

  The Cygwin API library found in the winsup
  subdirectory of the source code is also covered by
  the GNU GPL (with exceptions; see below). By default,
  all executables link against this library (and in the
  process include GPL'd Cygwin glue code). This means
  that unless you modify the tools so that compiled
  executables do not make use of the Cygwin library,
  your compiled programs will also have to be free
  software distributed under the GPL with source code
  available to all.

  Cygwin API Licensing Terms

  This is a copy of CYGWIN_LICENSE from the cygwin
  sources

  -----------------------------------------------------
  This program is free software; you can redistribute
  it and/or modify it under the terms of the GNU
  General Public License (GPL) as published by the Free
  Software Foundation; either version 2 of the License,
  or (at your option) any later version.

  This program is distributed in the hope that it will
  be useful, but WITHOUT ANY WARRANTY; without even the
  implied warranty of MERCHANTABILITY or FITNESS FOR A
  PARTICULAR PURPOSE. See the GNU General Public
  License for more details.

  You should have received a copy of the GNU General
  Public License along with this program; if not, write
  to the Free Software Foundation, Inc., 59 Temple
  Place - Suite 330, Boston, MA 02111-1307, USA.
  -----------------------------------------------------

                       *** NOTE ***

  In accordance with section 10 of the GPL, Red Hat
  permits programs whose sources are distributed under
  a license that complies with the Open Source
  definition to be linked with libcygwin.a without
  libcygwin.a itself causing the resulting program to
  be covered by the GNU GPL.

  This means that you can port an Open Source(tm)
  application to cygwin, and distribute that executable
  as if it didn't include a copy of libcygwin.a linked
  into it. Note that this does not apply to the cygwin
  DLL itself. If you distribute a (possibly modified)
  version of the DLL you must adhere to the terms of
  the GPL, i.e., you must provide sources for the
  cygwin DLL.

  See http://www.opensource.org/osd.html for the
  precise Open Source Definition referenced above.

  Red Hat sells a special Cygwin License for customers
  who are unable to provide their application in open
  source code form. For more information, please see:
  http://www.redhat.com/software/tools/cygwin/, or call
  866-2REDHAT ext. 3007
