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)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #385407
debian

00:00:15

 
  Build+Test
passed #385409
meson-clang

00:07:53

passed #385413
meson-clover

00:08:12

passed #385412
meson-main

00:02:27

passed #385408
meson-swr-glvnd

00:05:59

passed #385414
meson-vulkan

00:00:47

passed #385416
scons-llvm

00:06:39

passed #385415
scons-nollvm

00:03:11

passed #385410
scons-swr

00:08:19

passed #385411
scons-win64

00:05:57