Modular Boost C++ Libraries Request

[grafikrobot]

We are in the process of making B2 build changes to all of the B2 build files
to support "modular" consumption of the Boost Libraries by users. See this list
post for some details: https://lists.boost.org/Archives/boost/2024/01/255704.php

The process requires making a variety of changes to make each Boost library
independent of the super-project structure. But the changes do not remove the
super-project structure or the comprehensive Boost release. The changes make
solely make it possible, optionally, for users, like package manages, to easily
consume libraries individually.

Generally the changes include:

• Adding a libroot/build.jam.
• Porting any functionality from libroot/jamfile to libroot/build.jam.
• Moving boost-install declaration from libroot/build/jamfile is applicable.
• Adjusting other B2 build files in the library, like test/jamfile, as needed.
• Possible changes to C++ source files to remove includes relative to the
  super-project boostroot location.

Some examples of such changes:

• Fix test Jamfile.v2 to be location independent. interval#38
• Avoid references to root build project. ublas#189
• Fix old Phoenix header refs. local_function#10
• Avoid references to root project. preprocessor#57
• Add modular library level build file. callable_traits#194
• Add modular library level build file. compatibility#4
• Add modular library level build file. hof#228
• Add modular library level build file. preprocessor#56

We are asking how you would like us to handle the changes. We would prefer if
you allow the owners of the Boost.org GitHub project to make changes to B2
build files, as needed, to accomplish the changes. But understand
that you may want to manage the proposed changes yourself.

We previously sent emails to all known maintainers to fill out a form with their
preference. We are contacting you in this issue as we have not gotten a response
to that email. You can see the ongoing responses for that form and the responses
to these issues here https://github.com/users/grafikrobot/projects/1/views/6

We are now asking if you can reply directly to this issue to indicate your
preference of handling the changes. Please supply a response to this question
and close the issue (so that we can verify you are a maintainer).

How would you like the build changes to be processed?

1. Pull request, reviewed and merged by a BOOSTORG OWNER.
2. Pull request, reviewed and merged by YOU.
3. Other. (please specify details in the reply)

Also please indicate any special instructions you want us to consider. Or other
information you want us to be aware of.

Thanks you, René

[jeking3]

We should make sure that PR builds are passing before we do this. I have no preference whether you do them or someone else does. The changes will not modify logic so there is little risk to this.