packages.md: document depends build targets

depends/packages.md

@@ -181,3 +181,18 @@ For us, it's much easier to just link a static `libsecondary` into a shared
 static or dynamic `libseconday`, that's not our concern. With a static
 `libseconday`, when we need to link `libprimary` into our executable, there's no
 dependency chain to worry about as `libprimary` has all the symbols.
+
+## Build targets:
+
+To build an individual package (useful for debugging), following build targets are available.
+
+    make ${package}
+    make ${package}_fetched
+    make ${package}_extracted
+    make ${package}_preprocessed
+    make ${package}_configured
+    make ${package}_built
+    make ${package}_staged
+    make ${package}_postprocessed
+    make ${package}_cached
+    make ${package}_cached_checksum