formatting broken in czech (and others?)

Using boost 1.66.0 (Fedora 28, if that matters)

```
#include
#include
#include
#include
#include
#include

int main(int argc, char **argv) {
namespace pt = boost::posix_time;
pt::ptime now = pt::second_clock::local_time();
using Facet = boost::posix_time::time_facet;
std::stringstream ss;
char *fmt;

setlocale(LC_ALL, "");
fmt = nl_langinfo(D_FMT);

auto output_facet(new Facet(fmt));
ss.imbue(std::locale(std::locale(), output_facet));
ss << now;
std::cout << ss.str() << std::endl;
}
```
Compile and run.

```
[notting@nostromo: ~]$ g++ -Wall -o test_datetime test_datetime.c -lboost_date_time
[notting@nostromo: ~]$ LANG=cs_CZ.UTF-8 ./test_datetime
%-d.%-m.2018
```

boost is not working properly in the presence of the `-` modifier. It also appears to ignore the `O` and `E` modifiers (used in Persian and other languages), but those at least still substitute ascii numbers properly.