fix CAFFE2_BUILD_MAIN_LIB to the correct C10_BUILD_MAIN_LIB (#70848)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70848
This is the C10 library, it that's the main lib we are building
here. While here, use `local_defines` instead of `copts` for this
definition. Both `copts` and `local_defines` only apply to the
compilation units in the library, and not transitively.
ghstack-source-id: 146998039
Test Plan: We are relying on CI to verify this doesn't cause any problems.
Reviewed By: malfet
Differential Revision: D33429420
fbshipit-source-id: b3fc84c0588bd43346e3f9f77e851d293bde9428
