python3 python-config script generates invalid includes

[matthewlweber]

BPO	31713
Nosy	@moreati
Superseder	bpo-22140: "python-config --includes" returns a wrong path (double prefix)
Files	0029-python-config.sh-don-t-reassign-prefix.patch

^{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 2020-11-27.14:00:05.932>
created_at = <Date 2017-10-06.13:37:29.352>
labels = ['build']
title = 'python3 python-config script generates invalid includes'
updated_at = <Date 2020-11-27.14:00:05.932>
user = 'https://bugs.python.org/matthewlweber'

bugs.python.org fields:

activity = <Date 2020-11-27.14:00:05.932>
actor = 'iritkatriel'
assignee = 'none'
closed = True
closed_date = <Date 2020-11-27.14:00:05.932>
closer = 'iritkatriel'
components = ['Cross-Build']
creation = <Date 2017-10-06.13:37:29.352>
creator = 'matthewlweber'
dependencies = []
files = ['47195']
hgrepos = []
issue_num = 31713
keywords = ['patch']
message_count = 1.0
messages = ['303819']
nosy_count = 2.0
nosy_names = ['Alex.Willmer', 'matthewlweber']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '22140'
type = None
url = 'https://bugs.python.org/issue31713'
versions = ['Python 3.4']

[matthewlweber]

Related to https://bugs.python.org/issue22907

If building in a path that starts with /usr, the includedir=$(echo "@includedir@" | sed "s#^$prefix_build#$prefix_real#") assignment in the python-config.sh ends up having the path it processes ran through a path substitution once before this line is executed because the @includedir@ in the python-config.sh.in is set to the string '${prefix}/include'. ${prefix} is assigned just above includedir in python-config.sh to prefix=$(echo "$prefix_build" | sed "s#^$prefix_build#$prefix_real#")

I believe we need to update the includedir to

includedir=$(echo "@includedir@")

Or rename the prefix variable in python-config.sh so that there isn't a naming conflict if a string is passed in via @includedir@ with that variable.

Without fixing this you end up with multiple /usr substitutions in the includedir string, each replaced with the real path. ie resulting in an invalid path.