configure: make Python 2 vs Python 3 explicit
configure: make Python 2 vs Python 3 explicit

Since Python 3 may be the active interpreter (python), instead of Python
2, move away from the assumption that python is Python 2.

Figure out what "python" is, and then use that when python2 or python3
are absent.  This way we prefer the versioned binaries over the generic
one, which may change over time (e.g. on Gentoo: eselect python ...).

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb