MFH: r511176

gecko: don't require gconf on non-Gnome2 desktops

Take advantage of lazy bindings by not requiring runtime dependency.
DBus is required under Wayland (Gecko-specific) and for MIME handling
but GConf2 is useless outside of Gnome2. So, only use GConf2 if it's
already installed (similar to PulseAudio).

PR: 240323
Approved by: ports-secteam blanket