meson: enforce -Wundef

With a tiny bit of rigueur, this allows us to catch all #if typos and
missing #includes, as seen in the previous commit.

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
10 jobs for !1192 with wundef in 8 minutes and 34 seconds (queued for 2 seconds)
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