[issue35025] Compiling `timemodule.c` can fail on macOS due to availability warnings

[Maxime Belanger]

New submission from Maxime Belanger <max at dropbox.com>:

We build Python on macOS with `-Werror=unguarded-availability` and `-mmacosx-version-min=<xx>` to ensure `libpython` is binary-compatible with earlier versions of macOS. 

This can create problems when building some modules, including `timemodule.c`, which was recently altered to fix bpo-28081. The initial fix is inappropriate, because attempting to reference `CLOCK_REALTIME` et al when `HAVE_CLOCK_GETTIME` is unset (in our case, due to being too "new"), results in a compiler error:

```
./Modules/timemodule.c:1368:29: error: '_CLOCK_REALTIME' is only available on macOS 10.12 or newer
      [-Werror,-Wunguarded-availability]
    PyModule_AddIntMacro(m, CLOCK_REALTIME);
                            ^~~~~~~~~~~~~~
/usr/include/time.h:154:24: note: expanded from macro 'CLOCK_REALTIME'
#define CLOCK_REALTIME _CLOCK_REALTIME
                       ^~~~~~~~~~~~~~~
./Include/modsupport.h:78:67: note: expanded from macro 'PyModule_AddIntMacro'
#define PyModule_AddIntMacro(m, c) PyModule_AddIntConstant(m, #c, c)
```

A more correct patch (I'm attaching ours) is to only add the macros to the module if at least one of the three functions is defined. This should continue to work for the author of the original issue as well as fix our problem.

----------
components: Extension Modules, macOS
messages: 328030
nosy: Maxime Belanger, ned.deily, ronaldoussoren
priority: normal
severity: normal
status: open
title: Compiling `timemodule.c` can fail on macOS due to availability warnings
type: compile error
versions: Python 3.6, Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35025>
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