So it looks like I broke the ability to override --build-lib :(  Without
testing, I think you might also need to handle the cross-compile case -
the version may be the same, but the platform different.  I know
distutils is a PITA so might make this difficult, but it might be better
that we only set build_lib if the user didn't - ie, trust the user knows
what they are doing if they specify that option, even in these
'mismatched version/platform' cases.