building modules from 3.8.0rc1 fails in a venv when system 3.8 is present

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-10-07.13:27:24.317>
labels = ['3.8', 'build', '3.7']
title = 'building modules from 3.8.0rc1 fails in a venv when system 3.8 is present'
updated_at = <Date 2019-10-07.14:04:56.889>
user = 'https://github.com/pmp-p'

bugs.python.org fields:

activity = <Date 2019-10-07.14:04:56.889>
actor = 'vstinner'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Build']
creation = <Date 2019-10-07.13:27:24.317>
creator = 'pmpp'
dependencies = []
files = []
hgrepos = []
issue_num = 38393
keywords = []
message_count = 3.0
messages = ['354095', '354096', '354098']
nosy_count = 2.0
nosy_names = ['vstinner', 'pmpp']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue38393'
versions = ['Python 3.7', 'Python 3.8']