Set some meta.platforms

Also fix meta.platform -> meta.platforms in a few places.

pkgs/tools/system/at/default.nix

@@ -33,5 +33,6 @@ stdenv.mkDerivation {
     description = ''The classical Unix `at' job scheduling command'';
     license = stdenv.lib.licenses.gpl2Plus;
     homepage = http://packages.qa.debian.org/at;
+    platforms = stdenv.lib.platforms.linux;
   };
 }

pkgs/tools/system/cron/default.nix

@@ -20,8 +20,9 @@ stdenv.mkDerivation {
   '';
 
   preInstall = "mkdir -p $out/bin $out/sbin $out/share/man/man1 $out/share/man/man5 $out/share/man/man8";
-  
+
   meta = {
     description = "Daemon for running commands at specific times (Vixie Cron)";
+    platforms = stdenv.lib.platforms.linux;
   };
 }

pkgs/tools/system/which/default.nix

@@ -10,5 +10,6 @@ stdenv.mkDerivation {
 
   meta = {
     homepage = http://ftp.gnu.org/gnu/which/;
+    platforms = stdenv.lib.platforms.all;
   };
 }