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>

meson.build

@@ -841,7 +841,7 @@ c_args = []
 foreach a : ['-Werror=implicit-function-declaration',
              '-Werror=missing-prototypes', '-Werror=return-type',
              '-Werror=incompatible-pointer-types',
-             '-Wundef',
+             '-Werror=undef',
              '-Werror=format',
              '-Wformat-security',
              '-fno-math-errno',