CMake: Workaround for some export errors when doing a static build
This refuses to export/install the p3assimp/p3ptloader/p3txafile
libraries when building statically, because they depend on libraries
that are never installed, and static linkage would require all of the
dependencies to be available.
A more proper fix would involve installing these base libraries, but
only when doing a static build. I suspect nobody will ask for that,
so I haven't done it.