Dependencies like optional and smart_ptr will no longer support C++03
therefore the new minimum language level is C++11, and all the CI jobs
for both C++98 and C++03 have been removed.

Fixed use of the deprecated top level timer class as it was causing
build problems using warnings-as-errors.

Fixed -Wdeprecated-cast in group.hpp.
Fixed -Winvalid-source-encoding in sample_new_features.cpp.
Fixed -Wself-assign-overloaded in format_test2.cpp.

These changes also allow b2 to work properly when executed from
within the format directory.

* Make the library modular usable.

* Switch to library requirements instead of source. As source puts extra source in install targets.

* Add missing lib reference.

* Add requires-b2 check to top-level build file.

* Bump B2 require to 5.2

* Change all <source> references to <library>.

* Update copyright dates.

* Move inter-lib dependencies to a project variable and into the build targets.

* Update build deps.

This fixes boostorg#93

This will reduce maintenance burden going forward.