[Cmake] Can I use CMAKE_COMPILER_IS_GNUCXX

[Brad King]

> It may... never timed it.  'Course I can't imagine why there is a limit
> anyway.  I would guess that it would increase the memory usage of the
> compiler not necessary the speed.

The limit is to allow the compiler to detect infinite loops of
instantiation:

template <int I>
struct Foo: public Foo<I-1>
{
};

template <>
struct Foo<0>
{
};

typedef Foo<10> Foo10; // works fine due to sentinel at Foo<0>
typedef Foo<-1> FooN1; // infinate instantiation depth

The writers of the standard required only 17 (or so) as a limit as an
implementation minimum.  At the time, they didn't imagine these complex
template-metaprogramming techniques that have since been discovered.

MSVC doesn't implement a limit.  In fact, I've found that it is possible
to crash the IDE's editor by writing code like the above.  When the editor
goes to do lookup of a class's members for the bubble-help, it gets in an
infinite loop and eventually stack overflows!

-Brad