* Add option NGRAPH_EXPORT_TARGETS_ENABLE
Default value is TRUE
This option controls exporting and installing ngraph cmake config files.
Static library build only works with NGRAPH_EXPORT_TARGETS_ENABLE=OFF
for now.

* Turn off option for static nGraph lib.

* Register backends

* Add missing header files.
Co-authored-by: Scott Cyphers <diyessi@users.noreply.github.com>