gh-136097: Fix sysconfig._parse_makefile() (#136166)

* Fix potential infinite recursion.
* Fix a bug when reference can cross boundaries of substitutions, e.g.
  a=$(
  b=$(a)a)
* Fix potential quadratic complexity.
* Fix KeyError for undefined CFLAGS, LDFLAGS, or CPPFLAGS.
* Fix infinite recursion when keep_unresolved=False.
* Unify behavior with keep_unresolved=False for bogus $ occurred before
  and after variable references.