doc building doesn't check for breathe

[perarnau]

The configure.ac logic to detect if docs can be built does not check if the breathe python module is available, resulting in setups were doxygen and sphinx are detected (and thus docs are configured to be built) but the build fails because of a missing python module.

We should make sure that the detected sphinx setup is working, either by detecting the python package (ax_python_module?) or run a check on sphinx itself.