Rename some variables from FOO to FOO_CMD

Rename some variables from FOO to FOO_CMD
This fixes a problem with commands like gzip, where if $GZIP is exported
in the environment, then when make runs a command it'll put the Makefile
variable's value in the environment. But gzip treats $GZIP as arguments
for itself, so when we run gzip it thinks we're giving it "gzip" as an
argument.