ccache: enable tests

Let's hope this doesn't break all over the place. Works here.

pkgs/development/tools/misc/ccache/default.nix

@@ -1,19 +1,23 @@
-{stdenv, fetchurl, runCommand, gcc, zlib}:
+{ stdenv, fetchurl, runCommand, gcc, perl, zlib }:
 
 let
+  # TODO: find out if there's harm in just using 'rec' instead.
   name = "ccache-${version}";
   version = "3.2.2";
   sha256 = "1jm0qb3h5sypllaiyj81zp6m009vm50hzjnx994ril94kxlrj3ag";
+  doCheck = true;
 
   ccache =
 stdenv.mkDerivation {
-  inherit name;
+  inherit name doCheck;
   src = fetchurl {
     inherit sha256;
     url = "mirror://samba/ccache/${name}.tar.xz";
   };
 
-  buildInputs = [ zlib ];
+  # The test dependency on perl should be gone in the next release:
+  buildInputs = [ zlib ]
+    ++ stdenv.lib.optionals doCheck [ perl ];
 
   passthru = {
     # A derivation that provides gcc and g++ commands, but that