Description of problem:
It is currently impossible to define the DISTTAG tag using the %{disttag} macro.  We would like to utilize this tag to store module build information, in addition to DISTRIBUTION.

Version-Release number of selected component (if applicable):
rpm-4.14.1-9.fc28.x86_64

How reproducible:
Always.

Steps to Reproduce:
1. Define %disttag in your ~/.rpmmacros
2. Build an RPM
3. Check for its presence with rpm -q --qf '%{disttag}\n'

Actual results:
DISTTAG is not defined.

Expected results:
DISTTAG is defined.

Additional info:
This is supposedly going into 4.14.2 but, if possible, we would like to have this sooner as it's blocking MBS changes.