Use more informative "false" rather than "dummy"

The value does not really matter of course (hence why it was "dummy"
before), but it is more consistent and logical to set it to "false".
Enclose the autoconf IT in a profile

We activate this integration by default only if /usr/bin/autoreconf
is found.

This skips the IT by default on systems such as OS X, where autoconf
is not normally installed.