Actually it is designed to be compiled to a static lib before usage. It comes with a large number of predefined project files for a lot of IDEs and build systems (all possible Visual Studios, cmake, CodeBlocks, devcpp, ...) and their documentation starts with the chapter "Compiling AS".
But to be honest, I personally agree with charlie here. I don't really see the advantage of having a .lib/.a file over directly including their source code. After all, we have their source code in our repo anyways, and the lib is directly build from it, so for the final OD binary it doesn't matter - and we can't provide lib's for all possible compilers and systems.
But because I'm not very experienced with all this cross platform building/linking stuff, this opinion could be horribly wrong. So for the time being I stick to svenskmand's and oln's way of doing it.
edit: I just pushed the comit that makes MingW compiling again with AngelScript. For VC and linux system somebody else has to adjust it. With static linking to the AS lib the OD binary is over 5 MB big, with directly including the source it's only about 3 MB.