Help with making a port C11 compliant

[Montgomery-Smith, Stephen]

Here is my problem:
http://beefy2.nyi.freebsd.org/data/93amd64-default/414631/logs/GiNaC-1.7.0.log

the configure script says:
configure: error: Standard ISO C++ headers are missing

In the Makefile I have the line:
USES=           compiler:c++11-lang

Is there anything else I need to do?  I should add that seems to be a
FreeBSD-9 only problem - it builds fine on FreeBSD-10 and higher.

Thanks, Stephen