Meta: Improve build compatibility with non-GNU userspaces

We depend on GNU-specific du switch `--apparent-size`. Busybox has this
implemented, but as `-b` instead.
Another part of the build system uses `cp --preserve=timestamps`. This
can be replaced by `rsync -t`, and rsync is already used through the
file.

Thanks to those changes, Serenity can be built on a Busybox system,
without GNU coreutils.

Meta/shell_include.sh

@@ -82,3 +82,20 @@ get_number_of_processing_units() {
 
   ($number_of_processing_units)
 }
+
+# We depend on GNU coreutils du for the --apparent-size extension.
+# GNU coreutils is a build dependency.
+if command -v gdu > /dev/null 2>&1 && gdu --version | grep -q "GNU coreutils"; then
+    GNUDU="gdu"
+else
+    GNUDU="du"
+fi
+
+disk_usage() {
+    # shellcheck disable=SC2003,SC2307
+    expr "$(${GNUDU} -sbm "$1" | cut -f1)"
+}
+
+inode_usage() {
+    find "$1" | wc -l
+}