Change 3affcaa2b8 attempts to reset
purelib & platlib to any "install-dir" specified by the user in
setup.cfg. This code is used when we are installing wheels.
The problem with this is that distutils is *always* setting
"i.install_lib" -- even when the user specifies nothing. This has the
result of unconditionally setting purelib == platlib.
On some systems this results in .so's from the wheel getting installed
into /usr/lib/python2.7 (purelib) rather than /usr/lib64/python
(platlib). Because distribution-packaged libraries have installed
their .so into platlib, we can now have a situation where the new
pip-installed library is picking up an old .so from the distro package
... with predictably bad results.
This takes the approach of checking the configuration to see if the
user has overridden install-dir and only resetting the paths if they
have. The override case is covered by existing test-cases.
Closes#2940
d871bcd1bb