Properties of non abstract classes must have body
Stable versions of valac throw errors when properties of non abstract classes don't have body.
https://gitlab.gnome.org/GNOME/gxml/blob/master/gxml/StreamReader.vala#L56
'auto' private field is a good idea, but fails with older valac
build log as example : https://launchpadlibrarian.net/434793952/buildlog_ubuntu-bionic-i386.gxml-0.20_0.19.0+1676+201907270416-1~ubuntu18.04.1_BUILDING.txt.gz
Edited by Yannick Inizan