Spirit V2.5.5

There was no version bump and changelog for Boost V1.61.0-1.66.0 releases.

Fixed keyword directives compilation on C++11. GH#256 TRAC#11493
Fixed subrules compilation. GH#279 GH#259
Fixed subrules use after scope bug. GH#284 TRAC#5270
Fixed undefined behavior in sequential or operator. Underlying parsers order of execution was dependent on compiler. GH#310
Spirit was accidentally relaying on types typedef of boost::optional. The typedef was removed in the new optional implementation. GH#271 TRAC#12349
Fixed problems with BOOST_SPIRIT_NO_PREDEFINED_TERMINALS (attr_cast, confix, distinct, keywords, and flush_multi_pass). GH#314 TRAC#13311
Added missing copy assignment operator to hold_any. GH#361 TRAC#8268

Fixed alternate operator. GH#201 GH#271 TRAC#12349
Fixed signed integer overflow in real parser. GH#245
Fixed overflow problem in uint_parser<signed T>. GH#297
Fixed get_current_line, get_line_start, and added get_line_end. GH#304
Permutations parser were always initializing optionals. GH#319 TRAC#12473
Changed iterator concept static assert from ForwardIterator to ReadableIteratorConcept && ForwardTraversalConcept. GH#320
Added assertations to real parsers to ensure that the parse_frac_n from user defined real policy does not return negative values and also to prevent static analyzers false-positives. GH#358

Fixed alternate operator. GH#271
Fixed no_buffering_policy missing copy constructor. GH#261
The ostream_iterator was inheriting std::iterator which is deprecated in C++17. GH#345

Store id as promoted type to allow out-of-enum values. GH#247
Fixed C2001 'newline in constant' warning in generated lexers. GH#324 TRAC#11540