include strstream instead of strstream.h

[Benjamin Kosnik]

> No, the warning is not fine, because it pretends that C-headers <X.h> are
> deprecated, which is not true.

Here's the warning:

This file includes at least one deprecated or antiquated header. \
Please use the <X> header instead of <X.h> header for 'C++' 
includes and use the <cX> header instead of <X.h> header for 'C' 
includes. To disable this warning use -Wno-deprecated.

Gaby, you are correct in that the "C" <X.h> headers are compatibility 
headers, and not deprecated. Of course, the compatibility headers are not 
done correctly (or at all, actually), which was the point of this 
warning. Sorry if that's confusing.

Something like:

This file includes at least one deprecated or antiquated C++ header. \
Please consider using one of the 32 headers found in section 17.4.1.2 
of the C++ standard: this may include substituting the <X> header instead of 
<X.h> header for C++ includes, or using <sstream> instead of 
<strstream.h>. To disable this warning use -Wno-deprecated

Might be more accurate.

-benjamin