m4: add --fatal-warnings

Error out if m4 generates a warning. This will help detect and prevent
malformed macros.

See 855084960feb9e593a3bca288ab328a34a99ac57 for motivation.

Test: policy compiles
Test: Policy doesn't compile if 855084960feb9e593a3bca288ab328a34a99ac57
      is reverted.

Change-Id: Iee6b6273bc2a24b1220861fd662573e76001defc
2 files changed