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

premake/README-mingw.txt

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+MinGW requires both the MinGW system and MSYS.
+
+There is a script for generating a series of GNU makefiles targeted
+at MinGW on Windows. These makefiles will build the SDL library and
+test executables with static links to libgcc and the same features
+as the Visual Studio builds. That is, they have full OpenGL support
+and they have no dependency on MinGW.
+
+After generating the scripts, simply navigate to the directory in
+a MSYS terminal and execute:
+
+    make
+
+If you wish to clean the directory, you can use either the clean
+batch file, or call:
+
+    make clean
+
+The former will remove the actual makefiles and the latter will
+perform a typical clean operation. You can target specific
+build configurations as such:
+
+    make config=debug
+
+Verbosity is initially set to off. All verbosity controls is
+whether the resulting gcc and ar commands are printed to the
+console. You can enable verbose output by setting verbose to any
+value:
+
+    make verbose=1
+
+There is currently no install target, but that is intended
+eventually.
+
+Ben:
+There is no DirectX support currently, but you can use the
+command option '--directx' when generating the makefiles to
+explicitly force the DirectX dependency on. This may have
+undefined behavior, so use it cautiously.