(merge from 3.2) Issue #9516: avoid errors in sysconfig when MACOSX_DEPLOYMENT_TARGET is set in shell.

Without this patch python will fail to start properly when the environment
variable MACOSX_DEPLOYMENT_TARGET is set on MacOSX and has a value that is
not compatible with the value during Python's build. This is caused by code
in sysconfig that was only meant to be used in disutils.