Turn common code into a static library
Pretty much what it says on the tin. Right now, Odamex compiles "common" twice, once for the client and once for the server. If common was turned into a static library, we could get away with compiling it just once and save significantly on build time.
This would likely involve the creation of an "API" struct to facilitate calling functions through function pointers between library and executable along with a "clientside|serverside" flag that could be set based on which side of the connection you're on. Also, I'm sure there is some unexpected gnarlyness that would have to be resolved.