Pre-canned distribution / platform configurations

When the software is configured for a platform, since we run on so
many platforms, detecting the best sequence of configure options to
get it building is hard.

Instead we have a text file containing the recommended (often the
distributed) version of these parameters, one per distribution or
significant platform. These are interpreted by autogen.sh and
eventually end up passed to configure. Thus:

./autogen.sh --with-distro=LibreOfficeAndroid

might build a version tweaked for Android.