Вот выдержка из черновика стандарта (первое, что нашлось):
3 Change: A name of file scope that is explicitly declared const, and
not explicitly declared extern, has internal linkage, while in C it
would have external linkage
Rationale: Because const objects can be used as compile-time values in
C++, this feature urges programmers to provide explicit initializer
values for each const. This feature allows the user to put const
objects in header files that are included in many compilation units.
Effect on original feature: Change to semantics of well-defined fea-
ture.
Difficulty of converting: Semantic transformation
How widely used: Seldom
