EUPS_PYTHON

Forcing eups to always use the same python as the one used during eups install :

* Add an option '--with-python' to configure.ac, which will set a
  variable EUPS_PYTHON and make it default to /usr/bin/python when not
  given. That way, when the user installs eups they're guaranteed
  to pick up the system python, or they're *forced* to be explicit
  about which one they want used.

* Then, use this value to expand the @EUPS_PYTHON@ macros on
  various hash-bang (#!) lines in bin/ folder.

The logic behind this is that EUPS will always default to system python
(/usr/bin/python). If the user wants something different, they have to
be _explicit_ about it. And if someone wants to distribute EUPS via eups
(say, using eupspkg scripts), they'd configure it as:

  $EUPS_PYTHON=`eups admin show python`
  CONFIGURE_OPTIONS='--prefix=$PREFIX --with-python=$EUPS_PYTHON'