When an output has the hotplug_mode_update property (e.g. SPICE/QXL
virtual displays), auto-configure to the new preferred mode on hotplug
events instead of applying stored configuration from monitors.xml.

This allows VM guest resolutions to follow host window resizes, which
stopped working when spice-vdagent switched from reconfiguring displays
directly to sending hotplug events expecting the DE to handle them.

Fixes #78

Many of the plugins depend on the libX11 and libXi linker flags, but
they are missing.

With GNU libtool the -no-undefined flag is silently ignored so the build
succeeds, but with slibtool is correctly sets -Wl,--no-undefined in the
linker flags.

Gentoo issue: https://bugs.gentoo.org/922326