build: Add -Werror=undef by default, fix fallout

The main thing here is that a ton of stuff has happened in gnulib since we
imported `parse-datetime.y`. I cherry-picked a little bit of it, but that
upstream doesn't seem to build with `-Wundef`, so I just deleted some hunks.

(Note I reindented the warnings consistently)

Update submodule: libglnx

Closes: #1320
Approved by: jlebon

1 files changed, 0 insertions, 0 deletions

diff --git a/libglnx b/libglnx
-Subproject d15a3790074fd982f2611a5b450dea61052dfc0
+Subproject b36606b366d39c7ddb90ee21d622c0cb1da118e