* 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.
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.
fixed a NO_LOCALE build error the benchmark build discovered,
updated ignored argument handling and fixed build warnings in clang,
changed CI build scripts to build from top level of library
