abuild-sign: use pigz if available

This is similar to what we do in abuild. It improves performance on
multicore machines.

abuild-sign.in

@@ -15,6 +15,8 @@ if ! [ -f "$sharedir/functions.sh" ]; then
 fi
 . "$sharedir/functions.sh"
 
+gzip=$(command -v pigz || echo gzip)
+
 do_sign() {
 	local f i keyname repo
 	local openssl=$(command -v openssl || echo libressl)
@@ -37,7 +39,7 @@ do_sign() {
 		fi
 
 		tmptargz=$(mktemp)
-		tar -f - -c "$sig" | abuild-tar --cut | gzip -9 > "$tmptargz"
+		tar -f - -c "$sig" | abuild-tar --cut | $gzip -9 > "$tmptargz"
 		tmpsigned=$(mktemp)
 		cat "$tmptargz" "$i" > "$tmpsigned"
 		rm -f "$tmptargz" "$sig"