'to_string' is not a member of 'std'

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

I'm trying to buld 2017Q3 (which I consider supported) on NetBSD-6.1 (which I 
consider supported). However, building secutity/gpgme fails with
	error: 'to_string' is not a member of 'std'

Changing it's USE_LANGUAGES from c++ to c++11 makes the configure phase fail 
because it tries to run /lib/cpp.

So I built lang/gcc48. However, compiling a simple
	#include <string>

	int main()
	{
		std::to_string(0);
		return 0;
	}
with /usr/pkg/gcc48/bin/cc -std=c++11 gives the same error. Same with 
lang/gcc49 and lang/gcc5.
On a NetBSD-7 machine (which also has gcc 4.8.5), it works!

I must be doing something stupidly wrong, no?