If ninja is being used, force build_ext to run. (#40837)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40837
As ninja has accurate dependency tracking, if there is nothing to do,
then we will very quickly noop. But this is important for correctness:
if a change was made to a header that is not listed explicitly in
the distutils Extension, then distutils will come to the wrong
conclusion about whether or not recompilation is needed (but Ninja
will work it out.)
This caused https://github.com/pytorch/vision/issues/2367
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Test Plan: Imported from OSS
Reviewed By: zou3519
Differential Revision: D22340930
Pulled By: ezyang
fbshipit-source-id: 481b74f6e2cc78159d2a74d413751cf7cf16f592
