As discussed on the forums, to simplify the CMake code and avoid having to
manually specify dependencies this removes the hack to allow both static
and shared libraries to be built at the same time.
It introduces the new variable LIBRARY_TYPE which can be either STATIC or
DYNAMIC.
See: http://www.volumesoffun.com/phpBB3/viewtopic.php?p=3203#p3203
Due to the hack in which we enable the building of both static and
shared libraries CMake's automatic dependency sorting isn't working.
Manually specifying the dependency seems to solve the problem of
components building out of order.
This is for future consistency where ENABLE_ variables will denote optional
components of PolyVox and WITH_ variables denote optionally using features
provided by external libraries (e.g. OGRE compatibility).
