The offending code appears to have been introduced in r33090 and tweaked
in r37025, which both originated from Mark Hammond.

From the comments, there seems to be no reason why the build_lib
override should apply when the target version is specifically different
from the version being used to build the extension, hence I've attached
a patch which applies the override only when the versions are different
(for both bdist_wininst and bdist_msi).

This doesn't solve the problem that exists when the target and build
versions are different and build_lib has been overridden in setup.py (as
I understand is the case with cx_Oracle).

I've added Mark to the nosy list in the hope he might, as originator of
the problematic changes, be able to provide some insight.