Allow undefined libraries to fix CMake errors

[aporialiao]

Summary: This should get rid of the libtorch_cpu.so: undefined reference to logf@GLIBC_2.27'` etc.. we see in our cpp unit tests on Github CI See: https://fb.workplace.com/groups/1405155842844877/permalink/23944202491846891/

Note there is another error of undefined reference - but it's related to our library not installing google benchmark rather than something inside of torch.so file like the ones mentioned in post.

Differential Revision: D71862824