install_prereq.sh: Differentiate between Debian and Ubuntu.

The Debian-based distro version check assumed it was Debian,
which would in a shell error something like this at the
beginning of any builds on Ubuntu:

./scripts/install_prereq.sh: 179: [: Illegal number: 22.04

The direct cause of the error is because Ubuntu releases are
decimal instead of purely integer. However, the versioning
scheme is different, so this check is only appropriate for
Debian and is now targeted as such.

scripts/install_prereq.sh

@@ -175,8 +175,16 @@ if [ -f /etc/fedora-release ] || [ -f /etc/redhat-release ]; then
 fi
 
 if [ -f /etc/debian_version ]; then
-	DEBIAN_VERSION=$( lsb_release -a | grep "Release:" | xargs | cut -d' ' -f2 )
-	if [ $DEBIAN_VERSION -ge 11 ]; then
+	DISTRO_TYPE=$( lsb_release -a | grep "Distributor ID:" | xargs | cut -d' ' -f3 )
+	OLD_DEBIAN_DISTRO=0
+	if [ "$DISTRO_TYPE" = "Debian" ]; then
+		DEBIAN_VERSION=$( lsb_release -a | grep "Release:" | xargs | cut -d' ' -f2 )
+		if [ $DEBIAN_VERSION -le 10 ]; then
+			# Some packages aren't available with Debian 10
+			OLD_DEBIAN_DISTRO=1
+		fi
+	fi
+	if [ $OLD_DEBIAN_DISTRO -eq 0 ]; then
 		# Not available in Debian 10
 		PACKAGES_DEBIAN="$PACKAGES_DEBIAN libcrypt-dev" # crypt_r
 		# used for cal (included in menus.conf sample)