Add CLIENT_BUILD option

Need to tweak things a bit, especially in the engine library module.
Some code in the engine module may be required by server executables,
such as configuration objects. However, some engine code requires GLFW
and/or GLEW.

Configuration objects should be moved to core, since it is an important
feature to all types of applications, and only depends on PhysicsFS, a
core dependency (linked and included in all build types). GL-related code
can also be moved out of the engine module and into the graphics module.
The engine library module can house code that would be common amongst
certain application types, such as networking code (clients and servers),
or the application objects (suitable for client-type builds).