Fix CMake errors on systems where {Q/X}NNPACK is not supported (#35607)
Summary:
- add a couple of checks for USE_XNNPACK to disable additional
code paths if XNNPACK is not supported
When passing through the code paths where the platform checks
are made (cmake/Dependencies.cmake:89), if XNNPACK is not
supported, then the var FXDIV_SOURCE_DIR will not be
set. CMake emits the errors when add_directory is called and
FXDIV_SOURCE_DIR is empty.
see: https://github.com/pytorch/pytorch/issues/34606
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35607
Differential Revision: D20895645
Pulled By: seemethere
fbshipit-source-id: 3bd10cf89f0fb6825fdd6e1d52c71ee37c67b953
