compile fail on intel-linux-12.1.3.0x because of bug on explicit bool conversion

[viboes]

The following code fail to compile with intel-linux-12.1.3.0x

          typename std::basic_ostream<CharT, Traits>::sentry opfx(os);
          if (opfx)
          {

with the message

../../../boost/chrono/io/duration_io.hpp(130): error: expression must have bool type (or be convertible to bool)
            if (opfx)

even if sentry defines an explicit bool conversion.

A workaround to this issue is needed.

[viboes]

It is enough to do the explicit conversion

          typename std::basic_ostream<CharT, Traits>::sentry opfx(os);
          if (bool(opfx))
          {