Deb: Stop depending on empty transitional package dh-systemd

MariaDB Server still supports Ubuntu 16.04 "Xenial" until it goes EOL
in April 30, 2021. Thus we need to include a customization for backwards
compatibility.

This change is intended to be applied for all MariaDB versions still
supported, i.e. 10.2 to 10.6.

debian/autobake-deb.sh

@@ -106,6 +106,15 @@ then
   sed '/Package: mariadb-plugin-cassandra/,/^$/d' -i debian/control
 fi
 
+# From Debian Stretch/Ubuntu Bionic onwards dh-systemd is just an empty
+# transitional metapackage and the functionality was merged into debhelper.
+# In Ubuntu Hirsute is was completely removed, so it can't be referenced anymore.
+# Keep using it only on Debian Jessie and Ubuntu Xenial.
+if apt-cache madison dh-systemd | grep 'dh-systemd' >/dev/null 2>&1
+then
+  sed 's/debhelper (>= 9.20160709~),/debhelper (>= 9), dh-systemd,/' -i debian/control
+fi
+
 # Adjust changelog, add new version
 echo "Incrementing changelog and starting build scripts"
 

debian/control

@@ -5,7 +5,7 @@ Maintainer: MariaDB Developers <maria-developers@lists.launchpad.net>
 Build-Depends: bison,
                chrpath,
                cmake (>= 2.7),
-               debhelper (>= 9),
+               debhelper (>= 9.20160709~),
                dh-apparmor,
                dh-systemd,
                dpatch,