bug#56249: 29.0.50; Compilation buffer parsing "ERROR:" incorrectly

[Mattias Engdegård]

>      
> ERROR:/Users/salutis/src/AndroidSchemeExperiment/app/build/intermediates/incremental/debug/mergeDebugResources/stripped.dir/layout/item.xml:3:
>  AAPT: error: '16dpw' is incompatible with attribute padding (attr) dimension.

That's not a GNU message format and I don't think we should bend the current 
`gnu` rule more, which is complex as it is.
"ERROR" is not a program name, and moreover adding arbitrary space tolerance 
before the rule undos some of the work previously made to prevent bad matching 
performance in compilation-mode.

What about asking the authors to follow the GNU message format instead? Failing 
that, a new pattern could be used. It would be much simpler than `gnu`.