configure includes user CFLAGS when detecting pthreads support

GitHub fields:

assignee = 'https://github.com/Yhg1s'
closed_at = None
created_at = <Date 2017-10-12.03:33:29.150>
labels = ['3.8', 'build', '3.7']
title = 'configure includes user CFLAGS when detecting pthreads support'
updated_at = <Date 2017-10-12.09:37:14.341>
user = 'https://github.com/floppym'

bugs.python.org fields:

activity = <Date 2017-10-12.09:37:14.341>
actor = 'yan12125'
assignee = 'twouters'
closed = False
closed_date = None
closer = None
components = ['Build']
creation = <Date 2017-10-12.03:33:29.150>
creator = 'floppymaster'
dependencies = []
files = ['47215', '47216']
hgrepos = []
issue_num = 31769
keywords = []
message_count = 2.0
messages = ['304200', '304201']
nosy_count = 4.0
nosy_names = ['twouters', 'Arfrever', 'floppymaster', 'yan12125']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue31769'
versions = ['Python 2.7', 'Python 3.4', 'Python 3.5', 'Python 3.6', 'Python 3.7', 'Python 3.8']