Update NETGEN plugin support

- Updated NETGEN related CMake options
-- NETGEN_PLUGIN will enable plugin support
-- NETGEN_INTERNAL if set will use the internal sources, otherwise the system
   library will be detected and linked against.
- Added NETGEN plugin support as a separate library when MONOLITHIC_BUILD is off
- Tweaked BUILD_SHARED_LIBS to be an option instead of forcing to ON.