MinGW g++ `strnlen` was not declared in this scope

currently I'm trying to run some code that I received from lynda.com but it is spitting out an error. The file I'm trying to compile is as follows, only including the calls to strnlen (strc.cpp):

#include <cstdio>
#include <cstring>
#include <memory>

#include "strc.h"

...

strc::strc() : data(nullptr) {
    msg("default ctor");
}

strc::strc(const char * s) {
    size_t slen = strnlen(s, _maxlen);
    data = new char[slen + 1];
    data[slen] = 0;
    memcpy(data, s, slen);
    msg("cstring ctor");
}

strc::strc(const strc & f) {
    size_t slen = strnlen(f.data, _maxlen);
    data = new char[slen + 1];
    data[slen] = 0;
    memcpy(data, f.data, slen);
    msg("copy ctor");
}
...

To compile this I am running:

g++ -c strc.cpp -o strc.o -std=c++11

I've tried replacing cstring with string.h and I've even tried but std:: in front of strnlen. Any help in this matter would be greately appreciated