bug#25791: address argument to atomic operation must be a pointer to _Atomic type with 2.1.7 on macOS

[ilove zfs <ilovezfs@icloud.com>]

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On macOS 10.11 and 10.12 building with Xcode 8, guile 2.1.7 fails to compile with several errors saying "address argument to atomic operation must be a pointer to _Atomic type." I can avoid this by setting ac_cv_header_stdatomic_h=no.



Build logs here:
https://gist.github.com/ilovezfs/b18b7e6160e0a4c5da297cc430961f63
https://gist.github.com/57e1513e03ede26dc97c0f63eaad3a98

Build failure is
```
  CC       libguile_2.2_la-arrays.lo
  CC       libguile_2.2_la-async.lo
In file included from async.c:27:
../libguile/atomics-internal.h:37:10: error: address argument to atomic operation must be a pointer to _Atomic type ('gl_uint32_t *' (aka 'unsigned int *') invalid)
  return atomic_fetch_sub (loc, arg);
         ^                 ~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/8.0.0/include/stdatomic.h:149:43: note: expanded from macro 'atomic_fetch_sub'
#define atomic_fetch_sub(object, operand) __c11_atomic_fetch_sub(object, operand, __ATOMIC_SEQ_CST)
```
and several similar errors.

Previously (as of guile 2.1.4) I had been able to work around the issue without setting ac_cv_header_stdatomic_h=no with this patch:
https://raw.githubusercontent.com/ilovezfs/formula-patches/d2798a468346a7a28fbcd1c8aa3c1ccd5627b03f/guile/guile-atomic-type.patch

However, the patch is no longer sufficient since now void** is one the types needing _Atomic, which isn't allowed.



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