Lib/sysconfig.py expands non-variables

[lepaperwan]

BPO	34689
Nosy	@berkerpeksag, @serhiy-storchaka, @lepaperwan
PRs	bpo-34689: Prevent sysconfig._parse_makefile from expanding $${variables} #9362 bpo-34689: Prevent sysconfig._parse_makefile from expanding $${variables} #20439

^{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 = None
created_at = <Date 2018-09-14.20:11:33.198>
labels = ['type-bug', 'library', '3.10']
title = 'Lib/sysconfig.py expands non-variables'
updated_at = <Date 2020-07-06.08:47:02.118>
user = 'https://github.com/lepaperwan'

bugs.python.org fields:

activity = <Date 2020-07-06.08:47:02.118>
actor = 'terry.reedy'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2018-09-14.20:11:33.198>
creator = 'lepaperwan'
dependencies = []
files = []
hgrepos = []
issue_num = 34689
keywords = ['patch']
message_count = 1.0
messages = ['325393']
nosy_count = 3.0
nosy_names = ['berker.peksag', 'serhiy.storchaka', 'lepaperwan']
pr_nums = ['9362', '20439']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue34689'
versions = ['Python 3.10']

[lepaperwan]

Lib/sysconfig.py _parse_makefile needs to expand Makefile-style variables which includes $() ${} variables with a litteral $ being $$.

That last particular point is properly accounted for in the first half of the function when determining values that need expansion and those that don't. However, the second half that actually performs variable expansion doesn't.

Sample breaking input:
VAR1= VALUE1
VAR2= VALUE2
VAR3=$${VAR1} ${VAR2}

Output:
Expect: {'VAR1': 'VALUE1', 'VAR2': 'VALUE2', 'VAR3': '$${VAR1} VALUE2'}
Actual: {'VAR1': 'VALUE1', 'VAR2': 'VALUE2', 'VAR3': '$VALUE1 VALUE2'}

I'm working on patching this but I'm putting this here as feedback is welcome.

As far as I could tell, this is a side effect of b2b1217, which patched bpo-24705.