Added Ben Henning's GSoC2013 work: premake build system.

premake/README-cygwin.txt

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+There is a script in the Cygwin/build-scripts folder for generating a series of
+GNU makefiles for building the SDL2 project and some parts of its test suite.
+These work similarly to the MinGW makefiles, but the overall Cygwin project has
+significant limitations.
+
+The current project will not build correctly. It's experimental and has a lot of
+tweaking needed to be built. It was built successfully once, but it has not been
+maintained in any way.
+
+The Cygwin project is limited in that it is not expected to be able to run
+anything visual at all. It is not difficult to enable all of the visual tests
+and support (such as X11 support or OpenGL), but it is not a goal for this
+project. For the complexity of having a compatible desktop environment setup on
+Cygwin, it's assumed that will not be the case for most users of the generated
+Cygwin project. As a result, only the core tests and library are built for
+Cygwin, focusing on things like thread support, file operations, and various
+system queries and information gathering.
+
+The Cygwin directory does have automated tests to run through the tests
+supported by Cygwin. It also has separate build scripts for both debug and
+release builds, though this is assuming the GNU make utility is located in the
+user's PATH.
+
+The Cygwin project has no outstanding dependencies, since it is designed to be
+mostly minimalistic and just relied on the POSIX functionality provided by
+Cygwin.
+
+Like the other projects, you may cleanup the entire directory of any generated
+or built files using the clean script located in Cygwin/build-scripts.