setuptools_dso hardcodes to use its own build_ext in its setup.
We amend that build_ext and inject our own version carefully while
setup() is run - see 7ae8c4f3 (pyx.build: Allow to combine C and C++
sources in one extension). However, currently, if user uses cmdclass =
{'build_ext': ...} in his own setup, this will be ignored and
overwritten by golang.pyx.build.build_ext . One example where this
breaks things is wendelin.core which hooks into build_ext to first
configure/build CCAN before any extension:

https://lab.nexedi.com/nexedi/wendelin.core/blob/b26ba558/setup.py#L147-153

-> Fix it by taking into account what user could put into cmdclass.