Have `make clean` remove directories as well

But apply at least minimal safeguards from running `rm -rf /` or
similar catastrophic cases.

src/include/build.sh

@@ -77,7 +77,8 @@ clean() {
     # Clean up tree after building
     #
     test -f "$MKIT_LOCAL/built.lst" && {
-        <"$MKIT_LOCAL/built.lst" xargs -r rm -f
+        <"$MKIT_LOCAL/built.lst" grep -v -e '\.\.' -e ^/ \
+          | xargs -r rm -rf
         rm -f "$MKIT_LOCAL/built.lst"
         rmdir --ignore-fail-on-non-empty "$MKIT_LOCAL"
     }