conc059 fails on Darwin

Compile failed (exit code 1) errors were:

conc059_c.c:40:19: error:
     error: implicit declaration of function 'nanosleep' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
            int ret = nanosleep(&ts, &ts);
                      ^
1 error generated.
`clang' failed in phase `C Compiler'. (Exit code: 1)

*** unexpected failure for conc059(threaded1)

Visible in https://gitlab.haskell.org/ghc/ghc/-/jobs/596291, https://gitlab.haskell.org/ghc/ghc/-/jobs/596648