Check existence of caffe2::mkl target (#119945)
Fixes #118862
If libtorch is included multiply times in different sub-folders, linking caffe2::mkl may incur errors like
```
Cannot specify link libraries for target "caffe2::mkl" which is not built
by this project.
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119945
Approved by: https://github.com/ezyang
