The INSTALL and SYSTEM variables have
disappeared.
The SYSTEM variable used to specify the system on
which the software was being built, and the mk script
switched on this value to make its settings, making the assumption
that the platform reliably implied the compilers being used. This is
now redundant, since the autoconf-based system makes its decision, not
on the basis of what platform it discovers it is running on, but on
the basis of the tools and capabilities it dynamically discovers. This
variable is now simply ignored.
The INSTALL variable used to specify the installation
location. This role is now performed by the --prefix...=
option to the ./configure script. There is a major clash
of conventions here: the generated ./configure script
will look at the INSTALL variable and, if it is set,
expect it to name a program to use instead of the standard
/usr/bin/install program, to install the software after
building. You should almost certainly not have this variable
defined, and if it is defined by accident, to anything other than an
installation program, the buildsystem will almost certainly fail very
messily.
See the top-level README file for further details about
environment variables which affect the build.