configure fails to detect C11 atomic support on clang

[benjaminp]

BPO	34121
Nosy	@benjaminp, @miss-islington
PRs	bpo-34121: Fix detection of C11 atomic support on clang. #8288 [3.7] bpo-34121: Fix detection of C11 atomic support on clang. (GH-8288) #8289 [3.6] bpo-34121: Fix detection of C11 atomic support on clang. (GH-8288) #8290

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-07-16.00:05:26.675>
created_at = <Date 2018-07-15.23:47:57.615>
labels = ['3.8', 'build', '3.7']
title = 'configure fails to detect C11 atomic support on clang'
updated_at = <Date 2018-07-16.00:48:48.458>
user = 'https://github.com/benjaminp'

bugs.python.org fields:

activity = <Date 2018-07-16.00:48:48.458>
actor = 'benjamin.peterson'
assignee = 'none'
closed = True
closed_date = <Date 2018-07-16.00:05:26.675>
closer = 'benjamin.peterson'
components = ['Build']
creation = <Date 2018-07-15.23:47:57.615>
creator = 'benjamin.peterson'
dependencies = []
files = []
hgrepos = []
issue_num = 34121
keywords = ['patch']
message_count = 4.0
messages = ['321697', '321698', '321704', '321705']
nosy_count = 2.0
nosy_names = ['benjamin.peterson', 'miss-islington']
pr_nums = ['8288', '8289', '8290']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue34121'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[benjaminp]

clang supports C11 atomic APIs just fine but Python's configure fails to detect it:

configure:16529: checking for stdatomic.h
configure:16545: clang -pthread -o conftest conftest.c -lpthread -ldl -lutil >&5
conftest.c:389:5: error: _Atomic cannot be applied to incomplete type 'void'
_Atomic void *py_atomic_address = (void*) &value;
^
1 error generated.

Indeed, the atomic uninhabited type is not very useful. If we wanted an atomic pointer, we should write _Atomic(void *).

[benjaminp]

New changeset 15c7b2a by Benjamin Peterson in branch 'master':
bpo-34121: Fix detection of C11 atomic support on clang. (GH-8288)
15c7b2a

[miss-islington]

New changeset 3fc12f0 by Miss Islington (bot) in branch '3.7':
bpo-34121: Fix detection of C11 atomic support on clang. (GH-8288)
3fc12f0

[benjaminp]

New changeset 8487ef6 by Benjamin Peterson in branch '3.6':
[3.6] bpo-34121: Fix detection of C11 atomic support on clang. (GH-8290)
8487ef6