Updated build process to build a shared library on Linux and OS X

@IainNZ @mlubin

In order to call ECOS from the Julia wrapper in [ECOS.jl](https://github.com/JuliaOpt/ECOS.jl), we updated the build process to build it as a shared library. Currently we do it by patching the original files, but we think this could be useful for other users so it may be interesting to have the functionality upstream.

This PR adds a new task to the Makefile (`make shared`), and adds the flag -fPIC to CFLAGS (in order to make the compiled code suitable for dynamic linking). We have also added the generated files to the list of files to be removed on `make clean`.