Makefile: Fix shared library building on various OSes

Versioned on OS != Windows, and create a .dll.a implib on Windows

This change is used by the GStreamer project with its continuous integration system based around [Cerbero](http://cgit.freedesktop.org/gstreamer/sdk/cerbero) for the srtp GStreamer plugin. This change has been tested on Mac OS X, Linux (Ubuntu/Fedora/Debian), Windows 32-bit and 64-bit (cross-compiled, mingw), and Android (cross-compiled).