accumulators: fix long-standing dependency tracking bug. merge [80826], [80885], [80888] from trunk. fixes #7409

svn path=/branches/release/boost/accumulators/; revision=80927
comment typo, fixes #7510

svn path=/branches/release/boost/accumulators/; revision=81306
merge [83305] from trunk, fixes #7917

svn path=/branches/release/boost/accumulators/; revision=83391