bootstrapping a C++ compiler (was: Selecting a C++ compiler)

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

I'm starting a subthread for GCC bootstrapping issues.

EF> -- We need to assure gcc-inplace-math (and -nls) when building GCC as a 
EF>    build dependency, either by making that the default or temporarily 
EF>    overriding the options.
GT> That is the perhaps controversial point.
What's controversial about this? Without inplace-math, we install packages 
providing libraries compiled with a lower GCC version, which renders them 
unusuable, no?
Or do you think the nls part is controversial?

> So do you think we need two versions, the "real" gcc6, and the
> "bootstrap" gcc6?  Two packages from same source?  Something else?
I think the easiest solution would be to make lang/gccX use 
PKG_OPTIONS.gccX-bootstrap instead of PKG_OPTIONS.gccX if it's being 
bootstrapped.

The tricky part (it occurs to me) is what "it's being bootstrapped" means.
I think the answer is "it's the Single Version" (or, regarding C, "one of 
the two Single Verssions") AND "it's not installed as a package". So, if 
you like lang/gcc6 (supposing that's one of your Single Versions) with nls, 
you can build lang/gcc6 with that set of options, which will first build 
your Single Version with the bootstrap options and then use that to copile 
the final version.