GNUMakefile.in: Set GROUP_T when installing configuration files as root

GNUMakefile.in: Set GROUP_T when installing configuration files as root

... and there is no privoxy user available.

Prevents the install target from failing with:

     Installing templates to /tmp/etc/templates
     id: privoxy: no such user
     ******************************************************************
      WARNING! WARNING! installing config files as root!
      It is strongly recommended to run privoxy as a non-root user,
      and to install the config files as that user and/or group!
      Please read INSTALL, and create a privoxy user and group!
     *******************************************************************
     Installing configuration files to /tmp/etc
     Installing config as config.new
     usage: install [-bCcpSsUv] [-f flags] [-g group] [-m mode] [-o owner]
                    [-M log] [-D dest] [-h hash] [-T tags]
                    [-B suffix] [-l linkflags] [-N dbdir]
                    file1 file2
            install [-bCcpSsUv] [-f flags] [-g group] [-m mode] [-o owner]
                    [-M log] [-D dest] [-h hash] [-T tags]
                    [-B suffix] [-l linkflags] [-N dbdir]
                    file1 ... fileN directory
            install -dU [-vU] [-g group] [-m mode] [-N dbdir] [-o owner]
                    [-M log] [-D dest] [-h hash] [-T tags]
                    directory ...
     gmake: *** [GNUmakefile:865: install] Error 1