Interpret --disable-module-dirauth=no correctly

Currently, we treat --disable-module-dirauth=no as enabling the C macro, but disabling the Makefile variable.

Apparently lots of people make this mistake:

The most common mistake for this macro is to consider the two actions as action-if-enabled and action-if-disabled.

This is not the case!

Since using --disable-foo or --enable-foo=no are equivalent, for the macro, you cannot really use this macro with those meanings. 

https://autotools.io/autoconf/arguments.html

I don't know if we should backport this change, it just didn't work before, so maybe it should just go in master?