Make static internals ptr pybind version specific

Under gcc, the `static internals *internals_ptr` is shared across .so's,
which breaks for obvious reasons.

This commit fixes it by moving the static pointer declaration into a
pybind-version-templated function.

Author: jagerman

include/pybind11/cast.h

@@ -43,8 +43,11 @@ struct type_info {
     bool default_holder : 1;
 };
 
+template <int = PYBIND11_VERSION_MAJOR, int = PYBIND11_VERSION_MINOR>
+inline internals *&get_internals_ptr() { static internals *internals_ptr = nullptr; return internals_ptr; }
+
 PYBIND11_NOINLINE inline internals &get_internals() {
-    static internals *internals_ptr = nullptr;
+    internals *&internals_ptr = get_internals_ptr();
     if (internals_ptr)
         return *internals_ptr;
     handle builtins(PyEval_GetBuiltins());