* Make the library modular usable.
* Use searched-lib targets for external/OS/Windows libraries to avoid trying to refer to mangled names for them. And hence failing to find them.
* Use the OS temp dir for creating test dir to avoid putting dirs in random location based on where one runs tests from.
* Switch to library requirements instead of source. As source puts extra source in install targets.
* Updated dependencies.
* Add missing NO_LIB usage requirements.
* Add missing import-search for cconfig/predef checks.
* 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.
* Split b2 dependencies into public and private.
* Undo temp dir for test change.
* Make winapi dep target-os specific. And remove direct atomic dep as it is already handled by conditional rule.
* Remove bug targets from all build as that's not a regular project.
