Make sure to include the custom lua_neutralize_longjmp function
when using the system lua library.

diff -Nrup a/hslua.cabal b/hslua.cabal
--- a/hslua.cabal	2013-10-05 13:24:23.000000000 +0200
+++ b/hslua.cabal	2013-10-21 21:51:56.031113660 +0200
@@ -30,7 +30,8 @@ Library
   Exposed-modules:      Scripting.Lua, Scripting.Lua.ConfigFile
   Hs-source-dirs:       src
   if flag(system-lua)
-    Pkgconfig-depends:    lua
+    Pkgconfig-depends:  lua
+    C-sources:          src/ntrljmp.c
   else
     C-sources:          src/lapi.c, src/lauxlib.c, src/lbaselib.c, src/lcode.c,
                         src/ldblib.c, src/ldebug.c, src/ldo.c, src/ldump.c, src/lfunc.c,