makefile suffix naming conventions?

[gk]

Is there a naming convention for different kinds of makefiles, aside from 
*.d for dependency makefiles and occasionally *.mk?

I want to distinguish makefiles that may contain rules from those that 
never will and would like to follow any conventions, if there are any, for 
a makefile with only variable assignments, include directives, etc. but no 
rules.
- Greg Keraunen