Failure building some packages with gcc 6.4 - error: '::acosh' has not been declared

[David Brownlee <abs%absd.org@localhost>]

I've noticed some packages seem to fail under gcc 6.4 with errors
similar to below:

I've tried forcing c++03 or c++11 just to see but it doesn't seem to
change the behaviour.

USE_LANGUAGES=          c c++03

Would anyone have any ideas - thanks

David

gmake[4]: Entering directory
'/var/obj/pkg/fonts/harfbuzz/work/harfbuzz-1.8.1/src'
  CXX      libharfbuzz_la-hb-blob.lo
In file included from /usr/pkg/gcc6/include/c++/math.h:36:0,
                 from hb-private.hh:45,
                 from hb-blob.cc:33:
/usr/pkg/gcc6/include/c++/cmath:1105:11: error: '::acosh' has not been declared
   using ::acosh;
           ^~~~~