gzip: force shell shebang to /bin/sh

gzip: force shell shebang to /bin/sh

gzip's configure uses $SHELL to find a POSIX-compliant shell to put in
the shebang of its shell scripts (zcat, gzexe, ...). However, we set
$SHELL to /bin/bash in the Makefile, which may not be present on the
target. We do make sure that /bin/sh always points to a valid shell on
the target so we can use that.

The configure discovery is completely broken for cross-compilation. The
same $SHELL is used during the build (it is used by make to run the
commands in rules) and on the target. Also, the checks for a valid
shell use the host shell, not the target shell.

We could try to patch gzip to fix that, but the checks can anyway not
be run for the target shell, so we'll have to override it with a cache
value anyway. So we can just as well do exactly that, without patching.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reported-by: Pascal Speck <kernel@iktek.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>