Why +=?

Is there a guide for "+"? In the "porters-handbook" I have not found so far

You use += when the variable is defined before and you want to append to it.

Here, I cannot see the variable being defined before, so I see += and I wonder where the mistake is. Should it be =? Should it be += because this is a master port and some slave port can set it before? Did it use to be += because of some now defunct code in the Makefile?

The only variables for which you use += in a Makefile (if they are not defined before and need appending) are the variables that you can set in make.conf like CFLAGS, CXXFLAGS, LDFLAGS, that's it.

Thank you for your explanation. I think it's good to have the explanation of "+" and "?" could record in the handbook. Many have problems with it

%%LOCALBASE%%

%%LOCALBASE%%/etc

%%USERS%%

DIR is not defined.
%%DATADIR%%

I do not really agree, ?= and += are make(1) syntax, set if empty and append. You should be able to know which one to use by yourself.
The porter's handbook needs to stay a general guideline of how to do things, it must not, and will not become an ISO9001 abomination.

^.. technically this is probably %%PREFIX%% not %%LOCALBASE%% (as they are files of this port not a dependency).