LU-16382 spec: use pkgconfig() as appropriate.

LU-16382 spec: use pkgconfig() as appropriate.

pkgconfig() is preferred over explicit dependencies on libfoo-devel.
For SUSE, this is particularly needed for system and with SLE15-SP4
the OBS automatically includes "systemd-mini", but that conflicts with
"system".  Usig "pkgconfig(systemd) resolves this.

Also there is no need to have "Depends" for library packages.  These
are determined automatically from the result of the build.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Idfa4fdfd8bf060175b64a1991d3367024a368344
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49366
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: jsimmons <jsimmons@infradead.org>