Fix cext for Python 2; ensure C extensions build successfully

Fix cext for Python 2; ensure C extensions build successfully

The C extensions have been broken since cc718cccc0bf8a01abdf4068c
however CI did not find this, because the build degraded to
non-C extensions without failing.   Ensure that if cext is set,
there is no fallback to non-cext build if the C extension build
fails.

Repair C related issues introduced in cc718cccc0bf8a01abdf4068c.

As C extensions have been silently failing on 2.7 for some commits,
the callcounts also needed to be adjusted for recent performance-related
changes.  That in turn required a fix to the profiling decorator
to use signature rewriting in order to support py.test's
fixture mechanism under Python 2, usage introduced under profiling
in 89bf6d80a9.

Fixes: #5076
Change-Id: Id968f10c85d6bf489298b1c318a1f869ad3e7d80