Previously, the python.pri helper would try to query which python
binary on the system to use via the 'which' command.
Some systems do not have which (some Docker-based CI systems,
for example).
We now detect the presence of 'which'. If it is not available,
we try to use 'python' as the Python command. We also print a
warning about 'which' not being available -- as well as a message
pointing users to the MUMBLE_PYTHON environment variable.
Fixesmumble-voip/mumble#2748