meson: add -Wundef as additional warning flag

5 jobs for fix-autotools-build in 8 minutes and 16 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #226248
gst indent

00:00:17

passed #226247
manifest

00:00:20

 
  Build
passed #226251
build fedora x86_64

00:04:15

passed #226250
build nodebug fedora x86_64

00:04:23

failed #226249
build fedora x86_64

00:03:13