Fix build on macOS 10.13:

39dceaae MDEV-10983: TokuDB does not compile on OS X 10.12
Make use of a different function to get the current tid.
Additionally, librt doesn't exist on OS X. Use System library instead.

 storage/tokudb/PerconaFT/cmake_modules/TokuFeatureDetection.cmake | 4 +++-
 storage/tokudb/PerconaFT/portability/portability.cc               | 9 ++++++++-
 storage/tokudb/PerconaFT/portability/tests/test-xid.cc            | 9 ++++++++-
 storage/tokudb/PerconaFT/portability/toku_config.h.in             | 1 +
 4 files changed, 20 insertions(+), 3 deletions(-)