Build system: Use GZIP_ENV instead of GZIP

Build system: Use GZIP_ENV instead of GZIP

Older versions of gzip assume the GZIP environment variable contains
flags[1].  Building netsniff-ng on a system with such a version of gzip
will yield the following error:

  INST  netsniff-ng/netsniff-ng
  gzip -9 -c netsniff-ng.8 > netsniff-ng/netsniff-ng.8.gz
  gzip: gzip: No such file or directory

A practical alternative to redefining GZIP in the Makefile seems to be
to use the GZIP_ENV make variable[2] instead.

[1] The GZIP environment variable was deprecated in gzip version 1.7.
See https://www.gnu.org/software/gzip/manual/html_node/Environment.html
and https://git.savannah.gnu.org/cgit/gzip.git/tree/NEWS for more
information.

[2] https://www.gnu.org/software/automake/manual/automake.html#index-GZIP_005fENV

Signed-off-by: Daniel Roggow <daniel.roggow@rockwellcollins.com>