package/gzip: add host variant

package/gzip: add host variant

In case someone is building on a musl-based distro (Alpine), we do as
for the target variant, and force the fflush_stdin detection.

We however do not do the /bin/sh trick, because we are building
natively, so the shell check is working.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>