On the toplevel configure and build system

[DJ Delorie]

> 2. If you put directories from the GCC repository into your build, you 
> should expect GCC and its libraries to be built; toplevel should not 
> disable GCC on the grounds that GCC does not support a given target.  

I disagree.  We have a single combined gcc+binutils+etc internal tree
that's used for many targets; some support gcc, some do not.  I added
another "don't build gcc for this" just last week.

> If in future the src repository is split up into separate repositories for 
> different projects, this principle will apply in a more fine-grained way - 

I see no reason to stop people from building in a combined source tree
for multiple targets, and expecting it to work.