Fix setup.py with CFLAGS/LDFLAGS

[t20100]

This PR:

• fixes a compilation issue if there is 2 spaces in CFLAGS or LDFLAGS env. variables (occuring in new jupyter-slurm conda env). In this case an empty string is passed as an argument to the compiler as "".
• avoids the use of self.extensions and use ext instead since it is the currently handled Extension
• avoid use of ext_package and use the fully qualified name in the Extension: Without this, the _cImageD11 is advertised as a "top level" module (I don't know why, it shouldn't). To check: from importlib.metadata import packages_distributions; packages_distributions()['_cImageD11']

[t20100]

BTW, I don't think adding CFLAGS and LDFLAGS is needed: Those flags looks to be duplicated in the compiler calls. setuptools looks to already do the job.

[jonwright]

Looks good, thanks!