Work around cray++ compiler issue

It's still not clear as to what exactly makes it emit this error when compiling
string_to_integer:

CC-3059 crayc++: INTERNAL __C_FILE_SCOPE_DATA__, File = <pugixml>/src/pugixml.cpp, Line = 4524, Column = 4
  Expected no overflow in routine.

But a viable workaround for now is to exploit the knowledge that it uses
two-complement arithmetics and invert the sign manually.

Fixes #125.

src/pugixml.cpp

@@ -4521,7 +4521,14 @@ PUGI__NS_BEGIN
 		}
 
 		if (negative)
+		{
+			// Workaround for crayc++ CC-3059: Expected no overflow in routine.
+		#ifdef _CRAYC
+			return (overflow || result > minneg) ? ~minneg + 1 : ~result + 1;
+		#else
 			return (overflow || result > minneg) ? 0 - minneg : 0 - result;
+		#endif
+		}
 		else
 			return (overflow || result > maxpos) ? maxpos : result;
 	}