diff --git a/system/rdiff-backup/rdiff-backup.SlackBuild b/system/rdiff-backup/rdiff-backup.SlackBuild
index bf33653912..2151f73c17 100644
--- a/system/rdiff-backup/rdiff-backup.SlackBuild
+++ b/system/rdiff-backup/rdiff-backup.SlackBuild
@@ -25,12 +25,21 @@
 
 PRGNAM=rdiff-backup
 VERSION=1.2.8
-ARCH=${ARCH:-i486}
 BUILD=${BUILD:-1}
 TAG=${TAG:-_SBo}
 
-TMP=${TMP:-/tmp/SBo}
+# Automatically determine the architecture we're building on:
+if [ -z "$ARCH" ]; then
+  case "$( uname -m )" in
+    i?86) ARCH=i486 ;;
+    arm*) ARCH=arm ;;
+    # Unless $ARCH is already set, use uname -m for all other archs:
+       *) ARCH=$( uname -m ) ;;
+  esac
+fi
+
 CWD=$(pwd)
+TMP=${TMP:-/tmp/SBo}
 PKG=$TMP/package-$PRGNAM
 OUTPUT=${OUTPUT:-/tmp}