meson: $DESTDIR might be undefined

This causes an error with -u. Just add an empty fallback.

tools/meson-make-symlink.sh

@@ -3,9 +3,9 @@
 # this is needed mostly because $DESTDIR is provided as a variable,
 # and we need to create the target directory...
 
-mkdir -vp "$(dirname "${DESTDIR}$2")"
+mkdir -vp "$(dirname "${DESTDIR:-}$2")"
 if [ "$(dirname $1)" = . ]; then
-    ln -vfs -T "$1" "${DESTDIR}$2"
+    ln -vfs -T "$1" "${DESTDIR:-}$2"
 else
-    ln -vfs -T --relative "${DESTDIR}$1" "${DESTDIR}$2"
+    ln -vfs -T --relative "${DESTDIR:-}$1" "${DESTDIR:-}$2"
 fi

units/meson-add-wants.sh

@@ -6,14 +6,14 @@ unit="$3"
 
 case "$target" in
     */?*) # a path, but not just a slash at the end
-        dir="${DESTDIR}${target}"
+        dir="${DESTDIR:-}${target}"
         ;;
     *)
-        dir="${DESTDIR}${unitdir}/${target}"
+        dir="${DESTDIR:-}${unitdir}/${target}"
         ;;
 esac
 
-unitpath="${DESTDIR}${unitdir}/${unit}"
+unitpath="${DESTDIR:-}${unitdir}/${unit}"
 
 case "$target" in
     */)