btrfs-progs: fix build, manpage compression command

btrfs-progs: fix build, manpage compression command

man pages for btrfs-progs are compressed by gzip by default. In Makefile the
variable GZIP is use, this evaluates to 'gzip gzip' on my system. From man
gzip:

> The environment variable GZIP can hold a set of default options for gzip.
> These options are interpreted first and can be overwritten by explicit
> command line parameters.

So using any other variable name fixes this. Patch is attached.

Signed-off-by: Christian Hesse <list@eworm.de>
Signed-off-by: Gene Czarcinski <gene@czarc.net>