libmemcached: set meta.platforms

And a more precise license (BSD => 3-clause BSD).

pkgs/development/libraries/libmemcached/default.nix

···
 
   buildInputs = [ cyrus_sasl libevent ];
 
-  meta = {
+  meta = with stdenv.lib; {
     homepage = http://libmemcached.org;
     description = "Open source C/C++ client library and tools for the memcached server";
-    license = "BSD";
+    license = licenses.bsd3;
+    platforms = platforms.linux;
   };
 }