This manually builds shared libraries for a bunch of ports. Using
libtool would be preferable but that's currently broken so I'm
linking the shared libraries manually.
the opus and modplug music libraries.
Previously it wasnt compiling as we do not have ports of those
libraries. I have also changed the install location of the library so it
installs under /usr/include/SDL2 instead of /usr/local/include/SDL2.