Revert "NO-JIRA: CMakelists.txt check that ../python exists before building it."

This reverts most of commit r1574138. The changes were intended to simplify
handling of setup.py by letting it detect when things were out of date,
but even when there are no changes setup.py does touch some files:

changing mode of /home/andrew/Work/qpid/bld-ninja/src/tests/python/commands/qpid-python-trunning install_egg_info
removing /home/andrew/Work/qpid/bld-ninja/src/tests/python/qpid_python-0.27-py2.7.egg-info
writing /home/andrew/Work/qpid/bld-ninja/src/tests/python/qpid_python-0.27-py2.7.egg-info

This commit restores the previous logic that avoids running setup.py if not needed
but retains the if (EXISTS ${python_src}) check to avoid all this logic if
there is no python source directory available.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1574656 13f79535-47bb-0310-9956-ffa450edef68