bsl: Elide librt on macOS when building with CMake (#2214)
bsl: Elide librt on macOS when building with CMake (#2214)

`librt` does not exist on macOS and should not be added as a dependency
for `bsl`.
bloomberg/bde,che2/bde,bloomberg/bde,bloomberg/bde,bloomberg/bde,bloomberg/bde,che2/bde,che2/bde,che2/bde