Configuration with autoconf

One can now produce a Makefile.conf and a sourceme.sh
using autoconf. Just type:

./configure

Or maybe

./configure CXX=icpc

In case you want to use mpi, just use:

./configure CXX=mpic++

Everything should be smooth. I took care that the generated Makefile.conf
is compatible with those stored in configurations/. In this way,
one can always resort to a saved configuration file. Moreover,
jumping between git branches (without need of reconfiguring) will be
easier.

More info on the manual page (no need to use autoconf to compile
the manual and see that page)

Closes #46