Force inline gregorian::date::date(special_values) to improve perf

At least GCC 6.5.0, 7.4.0 and 8.3.0 fail to inline
gregorian::date::date(special_values) when constructing
posix_time::ptime and possibly other types. This leads to large
performance overheads for a operation that should ideally be a store of
a constant to some memory location.

include/boost/date_time/gregorian/greg_date.hpp

@@ -73,7 +73,7 @@ namespace gregorian {
       date_time::date<date,gregorian_calendar, date_duration>(rhs)
     {}
     //! Constructor for infinities, not a date, max and min date
-    explicit date(special_values sv):
+    BOOST_FORCEINLINE explicit date(special_values sv):
       date_time::date<date, gregorian_calendar, date_duration>(date_rep_type::from_special(sv))
     {
       if (sv == min_date_time)