build: Set G_LOG_DOMAIN to libchamplain
build: Set G_LOG_DOMAIN to libchamplain

"Libraries should define this so that any messages which they log can be
differentiated from messages from other libraries and application code"

https://developer.gnome.org/glib/stable/glib-Message-Logging.html#G-LOG-DOMAIN:CAPS

This enables us to filter debug messages to only see libchamplain
output — quite useful!

E.g.  G_MESSAGES_DEBUG=libchamplain CHAMPLAIN_DEBUG=view gnome-maps

https://bugzilla.gnome.org/show_bug.cgi?id=761368

StanciuMarius/Libchamplain-map-wrapping,StanciuMarius/Libchamplain-map-wrapping,StanciuMarius/Libchamplain-map-wrapping,GNOME/libchamplain,StanciuMarius/Libchamplain-map-wrapping,StanciuMarius/Libchamplain-map-wrapping,GNOME/libchamplain