build: do not allow unresolved symbols

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>
2 jobs for mr/libexec-weston in 2 minutes and 9 seconds (queued for 6 seconds)
latest
Status Job ID Name Coverage
  Container Prep
passed #408744
container_prep

00:01:10

 
  Build
passed #408745
build-native-meson

00:00:58