Make explicit that MAKEOBJDIRPREFIX and MAKOBJDIR must be set

Make explicit that MAKEOBJDIRPREFIX and MAKOBJDIR must be set
as environment variables and should not be set on make's command
line. They happen to work accidentially as command line variables
too when none of the sub-makes wants to play games with them (because
make is putting command line variables into the environment and will
find them there later on). Makefile.inc1 wants to change
MAKEOBJDIRPREFIX. In this case one cannot set it on the command line.