Fix compilation with libc++

Fix compilation with libc++

libc++ (http://libcxx.llvm.org/) is used on OS X with newer XCode.
The patch is from Benjamin Piwowarski <benjamin.piwowarski@lip6.fr>, I only
added more comments.
The changes regarding implicit conversion to bool of std::iostream work
because both the C++98 and C++11 standards guarantee that boolean evaluation
of streams returns !fail(). See e.g.
http://stackoverflow.com/questions/1334858/why-dont-iostream-objects-overload-operator-bool
for details.