cppNonPod pragma

The cppNonPod pragma should be used for non-POD importcpp types so that they work properly (in particular regarding constructor and destructor) for threadvar variables. This requires --tlsEmulation:off.

type Foo {.cppNonPod, importcpp, header: "funs.h".} = object
  x: cint
proc main()=
  var a {.threadvar.}: Foo