Permit user to suppress passing --prefix to setup.py

Permit user to suppress passing --prefix to setup.py

We change all invocations of setup.py as follows:

* use $(PYTHON) instead of `python' so that the user can specify
  an alternative python version if they need to.  If not set it
  defaults to `python' in Config.mk.

* pass --prefix=$(PREFIX) via a new make variable
  $(PYTHON_PREFIX_ARG).  This allows a user to suppress the
  --prefix=... argument entirely by setting PYTHON_PREFIX_ARG=''.
  This will work around the bug described here
     https://bugs.launchpad.net/ubuntu/+bug/362570
  where passing --prefix=/usr/local (which ought to have no effect as
  /usr/local is the default prefix) changes which subdirectory
  distutils chooses, and results in the files being installed in
  site-packages which is not on the default search path.

  Users not affected by this python packaging bug should not set
  PYTHON_PREFIX_ARG and their builds will not be affected.  (Provided
  PREFIX did not contain spaces.  People who put spaces in PREFIX are
  being quite optimistic.)

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>