lib: Factor in tiny bit of meta.platform checking

I need it in stdenv and release-lib, so that seems motivation enough.

lib/meta.nix

@@ -67,4 +67,23 @@ rec {
   */
   hiPrioSet = set: mapDerivationAttrset hiPrio set;
 
+
+  /* Check to see if a platform is matched by the given `meta.platforms`
+     element.
+
+     A `meta.platform` pattern is either
+
+       1. (legacy) a system string.
+
+       2. (modern) a pattern for the platform `parsed` field.
+
+     We can inject these into a patten for the whole of a structured platform,
+     and then match that.
+  */
+  platformMatch = platform: elem: let
+      pattern =
+        if builtins.isString elem
+        then { system = elem; }
+        else { parsed = elem; };
+    in lib.matchAttrs pattern platform;
 }