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.

Signed-off-by: Christian Hesse <mail@eworm.de>
Signed-off-by: David Sterba <dsterba@suse.cz>

Documentation/Makefile

@@ -45,7 +45,7 @@ MANPAGE_XSL = manpage-normal.xsl
 XMLTO = xmlto
 XMLTO_EXTRA =
 XMLTO_EXTRA = -m manpage-bold-literal.xsl
-GZIP = gzip
+GZIPCMD = gzip
 INSTALL ?= install
 RM ?= rm -f
 LNS ?= ln -sf
@@ -80,9 +80,9 @@ clean:
 	$(RM) *.xml *.xml+ *.8 *.8.gz
 
 %.8.gz : %.8
-	$(QUIET_GZIP)$(GZIP) -n -c $< > $@
+	$(QUIET_GZIP)$(GZIPCMD) -n -c $< > $@
 
-%.8 : %.xml 
+%.8 : %.xml
 	$(QUIET_XMLTO)$(RM) $@ && \
 	$(XMLTO) -m $(MANPAGE_XSL) $(XMLTO_EXTRA) man $<
 %.xml : %.txt asciidoc.conf