libstdc++: Three new warnings while building the library (patchincluded)

[Benjamin Kosnik]

Usually we use the canonical c++ style for casting, which is 
static_cast<to_type>(from_type)

I did this simple change.

In. 

thanks,
benjamin

> Since the third warning is generated by the line number 1133 of file
> basic_string.h, which does not exisist, basic_string.h consists only
> of 1029 line numbers, I do not know how to fix the problem.

yeah this is due to the char_traits change. Gaby probably didn't see it 
because char is unsigned char on solaris (? I think). 

-benjamin