Added Box2D

external/Box2D-2.3.1/Box2D/Building.txt

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+The Build folder contains custom made project files for Visual Studio and XCode.
+
+For other platforms you need to run premake in this directory. You can get premake here:
+http://industriousone.com/premake
+
+For example, on Linux, you would type:
+premake4 gmake
+
+This will create a gmake folder in the Build directory. From there you can run:
+make config="debug"
+
+If you have build problems, you can post a question here:
+http://box2d.org/forum/viewforum.php?f=7
+
+=============== OLD METHOD ====================
+
+Box2D uses CMake to describe the build in a platform independent manner.
+
+First download and install cmake from cmake.org
+
+For Microsoft Visual Studio:
+- Run the cmake-gui
+- Set the source directory to the path of Box2D on your PC (the folder that contains this file).
+- Set the build directory to be the path of Box2D/Build on your PC.
+- Press the Configure button and select your version of Visual Studio.
+- You may have to press the Configure button again.
+- Press the Generate button.
+- Open Box2D/Build/Box2D.sln.
+- Set the Testbed or HelloWorld as your startup project.
+- Press F5 or Ctrl-F5 to run.
+
+For Unix platforms, say the following on a terminal: (Replace $BOX2DPATH with the directory where this file is located.)
+	cd $BOX2DPATH/Build
+	cmake -DBOX2D_INSTALL=ON -DBOX2D_BUILD_SHARED=ON ..
+	make
+	make install	
+You might want to add -DCMAKE_INSTALL_PREFIX=/opt/Box2D or similar to the cmake call to change the installation location. make install might need sudo.