Build fails on macOS when compiler doesn't define __has_builtin

[jmroot]

BPO	42692
Nosy	@ronaldoussoren, @ned-deily, @jmroot, @miss-islington
PRs	bpo-42692: fix __builtin_available check on older compilers #23873 [3.9] bpo-42692: fix __builtin_available check on older compilers (GH-23873) #24090

^{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 2021-01-04.11:01:55.926>
created_at = <Date 2020-12-20.18:27:40.689>
labels = ['OS-mac', 'build', '3.9', '3.10']
title = "Build fails on macOS when compiler doesn't define __has_builtin"
updated_at = <Date 2021-01-04.11:01:55.926>
user = 'https://github.com/jmroot'

bugs.python.org fields:

activity = <Date 2021-01-04.11:01:55.926>
actor = 'ned.deily'
assignee = 'none'
closed = True
closed_date = <Date 2021-01-04.11:01:55.926>
closer = 'ned.deily'
components = ['Build', 'macOS']
creation = <Date 2020-12-20.18:27:40.689>
creator = 'jmr'
dependencies = []
files = []
hgrepos = []
issue_num = 42692
keywords = ['patch']
message_count = 4.0
messages = ['383437', '384318', '384319', '384320']
nosy_count = 4.0
nosy_names = ['ronaldoussoren', 'ned.deily', 'jmr', 'miss-islington']
pr_nums = ['23873', '24090']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue42692'
versions = ['Python 3.9', 'Python 3.10']

[jmroot]

The line in posixmodule.c that checks for __builtin_available is rejected by compilers that don't have __has_builtin. The second check needs to be in a nested #if.

[miss-islington]

New changeset df21f50 by Joshua Root in branch 'master':
bpo-42692: fix __builtin_available check on older compilers (GH-23873)
df21f50

[ned-deily]

New changeset eedeaef by Miss Islington (bot) in branch '3.9':
bpo-42692: fix __builtin_available check on older compilers (GH-23873) (GH-24090)
eedeaef

[ned-deily]

Thanks for the PR!