On macOS Sierra (OSX 10.12.1):

    $ ./configure --with-pydebug && make
    [... lots of output omitted ...]
    gcc -c -Wno-unused-result -Wsign-compare -g -O0 -Wall -Wstrict-prototypes    -std=c99 -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers   -I. -I./Include    -DPy_BUILD_CORE -o Python/random.o Python/random.c
    Python/random.c:97:19: warning: implicit declaration of function 'getentropy' is
          invalid in C99 [-Wimplicit-function-declaration]
                res = getentropy(buffer, len);
                      ^
    1 warning generated.

This is because OSX 10.12.1 has getentropy() but does not have
getrandom(). You can see this in pyconfig.h:

    /* Define to 1 if you have the `getentropy' function. */
    #define HAVE_GETENTROPY 1

    /* Define to 1 if the getrandom() function is available */
    /* #undef HAVE_GETRANDOM */

and this means that in Python/random.c the header <sys/random.h> is
not included:

    #  ifdef HAVE_GETRANDOM
    #    include <sys/random.h>
    #  elif defined(HAVE_GETRANDOM_SYSCALL)
    #    include <sys/syscall.h>
    #  endif

It's necessary include <sys/random.h> if either HAVE_GETRANDOM or
HAVE_GETENTROPY is defined.

New changeset 828251c2bccf by Ned Deily in branch '2.7':
Issue #28676: Prevent missing 'getentropy' declaration warning on macOS.
https://hg.python.org/cpython/rev/828251c2bccf

New changeset 0efd48d4c47c by Ned Deily in branch '3.5':
Issue #28676: Prevent missing 'getentropy' declaration warning on macOS.
https://hg.python.org/cpython/rev/0efd48d4c47c

New changeset e2faa8a22b69 by Ned Deily in branch '3.6':
Issue #28676: merge from 3.5
https://hg.python.org/cpython/rev/e2faa8a22b69

New changeset b31a7efd8b31 by Ned Deily in branch 'default':
Issue #28676: merge from 3.6
https://hg.python.org/cpython/rev/b31a7efd8b31

Thanks for the patch, Gareth!  Pushed for release in 2.7.13, 3.5.3, 3.6.0b4, and 3.7.0.