abseil needs a fix for macOS < 10.13: error: 'clockid_t' has not been declared

[barracuda156]

Turns out Ryan already ran into the problem: ​https://github.com/abseil/abseil-cpp/issues/1501 Upstream apparently can’t care less to fix anything. We need to find a fix on our end.

Otherwise multiple ports will be broken. Another example: ​https://github.com/macports/macports-ports/pull/19905#issuecomment-1679736275

In file included from /opt/local/include/absl/synchronization/mutex.h:74,
                 from /opt/local/include/absl/strings/internal/cordz_info.h:31,
                 from /opt/local/include/absl/strings/cord.h:91,
                 from /opt/local/include/absl/status/internal/status_internal.h:23,
                 from /opt/local/include/absl/status/status.h:59,
                 from /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_math_s2geometry/s2geometry/work/s2geometry-7773d518b1f29caa1c2045eb66ec519e025be108/src/s2/s2coder.h:23,
                 from /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_math_s2geometry/s2geometry/work/s2geometry-7773d518b1f29caa1c2045eb66ec519e025be108/src/s2/encoded_s2point_vector.h:31,
                 from /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_math_s2geometry/s2geometry/work/s2geometry-7773d518b1f29caa1c2045eb66ec519e025be108/src/s2/encoded_s2point_vector.cc:18:
/opt/local/include/absl/synchronization/internal/kernel_timeout.h:94:45: error: 'clockid_t' has not been declared
   94 |   struct timespec MakeClockAbsoluteTimespec(clockid_t c) const;
      |                                             ^~~~~~~~~
In file included from /opt/local/include/absl/synchronization/mutex.h:74,
                 from /opt/local/include/absl/strings/internal/cordz_info.h:31,
                 from /opt/local/include/absl/strings/cord.h:91,
                 from /opt/local/include/absl/status/internal/status_internal.h:23,
                 from /opt/local/include/absl/status/status.h:59,
                 from /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_math_s2geometry/s2geometry/work/s2geometry-7773d518b1f29caa1c2045eb66ec519e025be108/src/s2/s2coder.h:23,
                 from /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_math_s2geometry/s2geometry/work/s2geometry-7773d518b1f29caa1c2045eb66ec519e025be108/src/s2/s2point.h:27,
                 from /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_math_s2geometry/s2geometry/work/s2geometry-7773d518b1f29caa1c2045eb66ec519e025be108/src/s2/s1angle.h:28,
                 from /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_math_s2geometry/s2geometry/work/s2geometry-7773d518b1f29caa1c2045eb66ec519e025be108/src/s2/s2cell_id.h:43,
                 from /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_math_s2geometry/s2geometry/work/s2geometry-7773d518b1f29caa1c2045eb66ec519e025be108/src/s2/encoded_s2cell_id_vector.h:29,
                 from /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_math_s2geometry/s2geometry/work/s2geometry-7773d518b1f29caa1c2045eb66ec519e025be108/src/s2/encoded_s2cell_id_vector.cc:18:
/opt/local/include/absl/synchronization/internal/kernel_timeout.h:94:45: error: 'clockid_t' has not been declared
   94 |   struct timespec MakeClockAbsoluteTimespec(clockid_t c) const;
      |                                             ^~~~~~~~~