Now that all cases of unresolved symbols have been either fixed or worked
around pending for a proper fix, we can switch the project to disallow
unresolved symbols during build. This will help catch programming mistakes
earlier.

Note, that existing Meson build directories will not automatically apply this
change. If you have an existing build directory, you must issue

	meson configure -Db_lundef=true

in it.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>