So did some more learning over here.
OpenBSD 6.2's Boost port which is Boost 1.58.0, was built using GCC 4.2.1.
OpenBSD 6.3's Boost port which is Boost 1.66.0, is built using GCC 4.9.4.
this was effectuated by the older port's Makefile not having any COMPILER line, whereas the new one has a COMPILER line which contains "ports-gcc",
https://man.openbsd.org/bsd.port.mk#COMPILER .
