[build2] C++20

[breakspot reyes]

I already using those projects,Do I need gcc 11? Branch devel/c++-modules?(repository) 

> On Mar 19, 2021, at 7:26 AM, Boris Kolpackov <boris at codesynthesis.com> wrote:
> 
> breakspot reyes via users <users at build2.org> writes:
> 
>> Im having issues compiling modules, error: header file <iostream> cannot
>> be imported because it is not known to be a header unit
>> import <iostream>;
> 
> You will need to me more specific: which compiler/version you are
> using, whether you have enabled modules support in your project,
> etc.
> 
> Currently, your best modules-wise is GCC master. See this recent
> announcement for details:
> 
> https://build2.org/blog/build2-cxx20-modules-gcc.xhtml
> 
> For example of build2 projects that use modules, see this repository:
> 
> https://github.com/build2/cxx20-modules-examples/