libdw support is optional

This was no longer optional, leading to deadcode. This regression was
found trying to fix the unwind variant in cerbero.
3 jobs for libdw-is-optional in 7 minutes and 17 seconds (queued for 1 second)
Status Name Job ID Coverage
  Preparation
passed manifest #53748

00:00:25

 
  Build
passed build fedora x86_64 #53749

00:03:34

 
  Full Builds
passed build android arm64 api28 #53750

00:03:17