mpich @4.1 is broken on <10.12: fatal error: os/lock.h: No such file or directory

[barracuda156]

In file included from ./include/ofi_osd.h:60,
                 from ./include/ofi_abi.h:38,
                 from ./include/ofi.h:49,
                 from src/log.c:41:
./include/osx/osd.h:170:10: fatal error: os/lock.h: No such file or directory
  170 | #include <os/lock.h>
      |          ^~~~~~~~~~~
compilation terminated.
make[3]: *** [src/libfabric_la-log.lo] Error 1
In file included from ./include/ofi_osd.h:60,
                 from ./include/ofi_abi.h:38,
                 from ./include/ofi.h:49,
                 from ./include/ofi_util.h:57,
                 from src/fabric.c:46:
./include/osx/osd.h:170:10: fatal error: os/lock.h: No such file or directory
  170 | #include <os/lock.h>
      |          ^~~~~~~~~~~
compilation terminated.
make[3]: *** [src/libfabric_la-fabric.lo] Error 1
In file included from ./include/ofi_osd.h:60,
                 from ./include/ofi_abi.h:38,
                 from ./include/ofi.h:49,
                 from src/fi_tostr.c:45:
./include/osx/osd.h:170:10: fatal error: os/lock.h: No such file or directory
  170 | #include <os/lock.h>
      |          ^~~~~~~~~~~
compilation terminated.
make[3]: *** [src/libfabric_la-fi_tostr.lo] Error 1
In file included from ./include/ofi_osd.h:60,
                 from ./include/ofi_perf.h:40,
                 from src/perf.c:41:
./include/osx/osd.h:170:10: fatal error: os/lock.h: No such file or directory
  170 | #include <os/lock.h>
      |          ^~~~~~~~~~~
compilation terminated.

This commit breaks the build: ​https://github.com/ofiwg/libfabric/commit/476c8aeab92652f9ee889fa51f8a466b2083c90a Function in question exists only on 10.12+: ​https://developer.apple.com/documentation/os/os_unfair_lock