[build2] C++20

[Boris Kolpackov]

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/