bpo-34689: Prevent sysconfig._parse_makefile from expanding $${variables}

[lepaperwan]

As described in bpo-34689, sysconfig._parse_makefile recognizes $${variable} as a variable to expand in the second half of the _parse_makefile function.
This pull-request aims to fix that by splitting strings on $$ before looking for variables in them and restoring the $$ in values that require more processing.
There is no change to the documentation as this is a bug-fix.

https://bugs.python.org/issue34689

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