Tarek:

Yes, failure to build an extension module for any reason should
definitely cause "build" to fail by default.  However, many Python
packages seem to come with optional extension modules, typically for a
performance speed-up.  simplejson, zope.interface, and twisted all do
this (these are three packages that allmydata-tahoe depends on).  My own
pyutil package used to do this, but I eventually just removed the
optional C extension module in order to avoid this problem.  So, for
those it would be good if the package setup.py could specify that an
extension module is optional and that failure to build it for any reason
does not mean failure of the "build".

Note that twisted's setup.py already attempts to do this for certain of
its extension modules, and if I understand correctly it is able to
detect and handle failure to build if gcc fails, but not if gcc is absent.