support/dependencies: add a check for a suitable gzip

support/dependencies: add a check for a suitable gzip

Recently, some hash mismatch have been reported, both by users as well
as autobuilder failures, about tarballs generated from git repositories.

This turned out to be caused by users having the 'gzip' command somehow
aliased to 'pigz' (which stand for: parallel implementation of gzip,
which takes advantage of multi-processor system to parallelise the
compression).

Unfortunately, the output of pigz-compressed archives differ from that
of gzip (even though they *are* valid gzip-compressed streams).

Add a dependency check that ensures that gzip is not pigz. If that is
the case, define a conditional dependency to host-gzip, that is used as
a download dependency for packages that will generate compressed files,
i.e. cvs, git, and svn.

Fixes:
    http://autobuild.buildroot.org/results/330/3308271fc641cadb59dbf1b5ee529a84f79e6d5c/

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Marcin Niestrój <m.niestroj@grinn-global.com>
Cc: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>