Improve library handling.
* new/toolset.jam
(flags): Add 'unchecked' parameter, too allow declaring flags for other
modules.
* new/builtin.jam: Don't declare LIBRARY_PATH_OPTION.
* new/borland.jam: Use flags to declare LIBRARY_PATH_OPTION and LIBRARY_OPTION.
* new/msvc.jam: Use flags to declare LIBRARY_PATH_OPTION and LIBRARY_OPTION.
[SVN r18222]
Vladimir Prus