> I just pulled the .tar.bz2 archive and compiled it with no issue.
In that case I suspect you have CFLAGS, CPPFLAGS and/or LDFLAGS set in your environment, or you are using a compiler with non- standard include/library paths. Default cc (clang) does not search in /usr/local by default (at least on 11.2, but that did not change with the update). Basically, the port's CONFIGURE_ARGS are there for a reason...
I compiled the standard archive and noticed that it used GCC 6.4.0 - and I think it does include /usr/local/include in the standard header files path. I checked the environment and there are no custom CFLAGS/CPPFLAGS/LDFLAGS. My default shell is csh (I doubt it makes any difference).
My test machine was a FreeBSD 11.2, if that matters.