Work around a compiler bug on MSVC.

Work around a compiler bug on MSVC.

The two 'reg_info' declarations introduced in f271f67d2f turned out
to confuse MSVC: It thinks that these declarations are definitions
and then complains about a missing default ctor (error C2512) and
about duplicate definitions later in the .cpp files (error C2766).

But the standard says in [temp.expl.spec] §13:
An explicit specialization of a static data member of a template or
an explicit specialization of a static  data member template is a
definition if the declaration includes an initializer; otherwise, it
is a declaration. [Note: The definition of a static data member of a
template that requires default-initialization must use a braced-init-
list:
    template<> X Q<int>::x;     // declaration
    template<> X Q<int>::x ();  // error: declares a function
    template<> X Q<int>::x { }; // definition
--end note]
There is no initializer at lst.h:35 and at exprseq.h:35. Hence, this
is merely a declaration and the compiler's error message is wrong.
Let's work around by #ifndef'ing the two lines for silly MSVC.