setup.py aborts build if cannot compile a C extension module

[ivank]

Twisted's setup.py should keep going if it can't build any C extension module. It should print some kind of warning, of course, like zope.interface's setup.py does.

There is a problem with exit codes, though: someone may want the build to stop if a module they need fails to build. I (hastily) propose some kind of --needcexts=x,y,z option, and if any of those modules fail to build, exit with a non-0 code.

[glyph]

I don't know if this is quite a duplicate, but see #3586.

[davidsarah]

I think it's a duplicate.