abuild: chdir to $builddir if it exists

chdir to $builddir before running prepare, build, package or check.

abuild.in

@@ -594,6 +594,12 @@ runpart() {
 	local part=$1
 	[ -n "$DEBUG" ] && msg "$part"
 	trap "die '$part failed'" EXIT
+	if [ -d "$builddir" ]; then
+		case "$part" in
+			prepare|build|package|check)
+				cd "$builddir";;
+		esac
+	fi
 	$part
 	trap - EXIT
 }