Why is GNAT (via GCC 4.7) a prerequisite for building GCC?

In the GCC installation prerequisites page, it says:

GNAT

In order to build GNAT, the Ada compiler, you need a working GNAT compiler (GCC version 4.7 or later).

Questions:

1. Is it a bit strange for GCC to require an Ada compiler, when it already requires a C++ compiler? I mean, GCC is not written in Ada AFAICR.
2. This means that if I have a non-GNU C/C++ compiler, or a pre-4.7 GCC, I can't install a newer GCC directly. And this means I can't install a newer GCC on a full-blown Linux distribution from, say, 7-8 years ago or so - even if it had the latest and greatest packages, including possibly even GNAT.

I'm not asking this as a rant, what I'm trying to understand is what the cause is, in terms of the structure of the code of GCC or dependent libraries, that's behind this requirement.