Merged Boost library build system changes from the trunk.
Covers the following trunk revisions:
--- [78934] ---
Minor stylistic changes made to the boostcpp.jam Boost library build module - wrapped comments and some code lines to 80 characters, removed duplicate imports, ordered imports alphabetically, corrected comment & end-user message typos, removed redundant quoting & trimmed trailing spaces.
--- [78935] ---
Made Boost library build script's '--with-...' & '--without-...' parameter checking more strict so e.g. '--run--with--system' is not interpreted as one of those options.
--- [78936] ---
Minor stylistic changes.
--- [78937] ---
Boost library build system code cleanup - removed a non-existing 'explicit' target being declared as explicit.
--- [78938] ---
Corrected a syntax error in Boost library's build system that was preventing unversioned library targets from being staged or installed on Windows & Unix when versioned layout is used. Closes ticket #6027 (https://svn.boost.org/trac/boost/ticket/6027).
--- [78939] ---
Boost library build system code cleanup - refactored the make-unversioned-links() rule.
--- [79300] ---
--- [79301] ---
Disabled generating unversioned library targets on Windows & Unix with requested versioned layout as the Boost library build has been reported to fail with this code enabled for Boost library release 1.50. See related Boost library development mailing list thread at http://lists.boost.org/Archives/boost/2012/06/194312.php for more detailed information.
