Makefile: Fix to work even with GZIP environment variable set.

Makefile: Fix to work even with GZIP environment variable set.

The rule here was written to assume that if the GZIP environment
variable was set that it would be the gzip binary to execute,
(similar to the CC and CXX variables). But GZIP is actually used
to pass arguments to gzip, so we have to use a different name.