Parsing shared library name fails on MacOS

[seanpmorgan]

Since https://github.com/tensorflow/tensorflow/commit/7efc61175c540a56b03e829ec917ce9efc1f06f9 custom-op's configure script will no longer properly parse the shared library name for macos TensorFlow: https://github.com/tensorflow/custom-op/blob/master/configure.sh#L94

As a quick fix in Addons we just altered the string parsing to be OS dependent, but a more elegant solution is probably possible. I tried passing the linkflag to copts but ran into issues without copying the library to the bazel build dir as a GENRULE.

Happy to submit a string parsing PR to fix to this repo; or modify the Addons script to match a better solution that comes from this.