Makefile for building

I'm not sure if this is the best way, but it is _a_ way.

The Makefile has a target `liblz4.so`, which builds the library you want, after compiling the object files from the submodules.

rustc is told to pass the list of object files to the linker which links them all together into the library you want. The actual output is some library with a hash in its name, so it also then touches the expected target. You can remove that though.

This produces a single binary library with all the C code also compiled in. It's a little hacky, but fairly sound anyway.