how to specify a default configuration in a library's dub.json? (i.e if the use doesn't not have subConfigurations)

[mw]

Hi,

I just did a build fix, and added two configurations to thrift's 
dub.json (whose   "targetType": "library")

https://github.com/apache/thrift/pull/2397/files

I want to know how to specify a default configuration in the 
library's dub.json? (i.e if the use doesn't not have 
subConfigurations).

Then the existing library user does not have to change his app's 
dub.json when he do `dub upgrade`.

Thanks.