rpm: Set buildroot for rpmbuild

So that older distributions can be used

functions-rpm.sh

@@ -116,9 +116,17 @@ get_rpmbuild() {
         --define "vendor $ICINGA_BUILD_BRANDING_VENDOR" \
         --define "dist $dist" \
         --define "_topdir ${WORKDIR}/build" \
-        --define "buildroot %{_buildrootdir}/%{name}" \
         "$@"
     )
+
+    source /etc/os-release
+    if [ "$ID" != sles ] && [[ "$VERSION" != 11.* ]]; then
+      RPMBUILD+=(
+        --define "_buildrootdir %{_topdir}/BUILDROOT" \
+        --define "buildroot %{_buildrootdir}/%{name}" \
+      )
+    fi
+
     declare -p RPMBUILD
 }