How to turn off warnings as errors

[Chris Piker]

Hi GDC

I'm testing the new gdc version 12.1 by compiling vibe.d and 
various other packages.  One problem that tripping me up when 
running commands like:
```
cd memutils
dub build --compiler=gdc-12
```
is that all deprecation warnings are treated as errors.  (For my 
local build of gdc, '-12' has been added as a program suffix.)

Most of the messages are similar to the following:
```
source/memutils/rbtree.d:734:9: error: Usage of the ‘body’ 
keyword is deprecated. Use ‘do’ instead. [-Werror=deprecated]
```

These are dub packages and I don't see how to set '-Wno-error' 
from the dub command line. Since I'm reluctant to change every 
dub.json file that I encounter, is there a global flag that will 
turn off -Werror for gdc?

Thanks for the advice (and thanks for gdc),