nix: update default.nix

* reduce src fileset to avoid unneccessary rebuilds
* test symbol visibility

default.nix

@@ -19,10 +19,13 @@ with builtins;
 with lib;
 
 let
-  src = ./.;
-  version = readFile "${src}/VERSION";
+  version = readFile ./VERSION;
 in stdenv.mkDerivation {
-  inherit src version;
+  src = with fileset; toSource {
+    root = ./.;
+    fileset = unions [ ./VERSION ./GNUmakefile ./bemenu.pc.in ./scripts ./lib ./man ./client ];
+  };
+  inherit version;
   pname = "bemenu";
 
   strictDeps = true;
@@ -64,6 +67,11 @@ in stdenv.mkDerivation {
     done
   '';
 
+  doCheck = stdenv.isLinux;
+  checkPhase = ''
+    make check-symbols
+  '';
+
   meta = {
     homepage = "https://github.com/Cloudef/bemenu";
     description = "Dynamic menu library and client program inspired by dmenu";