Makefile, ${VARIABLE}= vs. ${VARIABLE}?=

[Helmut Schneider]

Chris Rees wrote:

> On 13 February 2011 14:14, Helmut Schneider <jumper99 at gmx.de> wrote:
> > [helmut at BSDHelmut ~]$ grep -ir latest /usr/ports/www/apache2?
> > /usr/ports/www/apache20/Makefile:LATEST_LINK=   apache20
> > /usr/ports/www/apache22/Makefile:LATEST_LINK?=  apache22
> > [helmut at BSDHelmut ~]$
> > 
> > So, what's the difference between e.g. "LATEST_LINK=" and
> > "LATEST_LINK?="?
> 
> ?= can be overridden in make.conf or on the command line.

Is it up to the maintainer to decide which variables shall be
overwritable, or are there rules?