Allow fstream ctor to take std::string arguments

[Gabriel Dos Reis]

While I would -not- want us focus on extensions, rather than trying to
implement what is *already* mandated by the standard, I think the
fstream not taking a std::string was such an obvious mistake that your
patch is probably OK for 3.1.  But, please, please, let's first try to
implement what is already there.

|     #define _GLIBCPP_STRINGS_IN_IOSTREAM_CTORS

	#define _GLIBCPP_EXTENSIONS

?

-- Gaby