sort out build/link problems

[johnnyquest]

In order to properly build it for Houdini, all alembic and -related includes and link libraries should be provided by SideFX with the Houdini install. (Especially on windows)

[johnnyquest]

Now almost everything is provided by SideFX (except that one has to build the alembic libs by oneself -- but using the other required libraries (hdf5, etc) from the Houdini installation works).

IMPORTANT NOTE:

To properly build on windows without symbol visibility problems, build your alembic libraries with the following #define-s: HDF5USEDLL, OPENEXR_DLL, PLATFORM_WINDOWS.