Some trivial fixes to make Nix working on Cygwin again

These changes allows me to build and run Nix on Cygwin (both i686-cygwin and x86_64-cygwin) again, if I compile it as follows:

./configure --disable-perl-bindings
make BUILD_SHARED_LIBS=0
make BUILD_SHARED_LIBS=0 install

I couldn't find any details on its deprecation, but according to some sources I have investigated earlier -std=c++0x means that the compiler should use strict ANSI/ISO C++ mode.

Most f*open() library calls are ISO compliant, but not fdopen() as far as I could see. If I compile the sources with -std=c++0x parameter on Cygwin, then I get an error saying that fdopen() is not declared in this scope. 

Changing the gcc parameter to -std=gnu++0x does seem to allow the sources to be compiled.

More info:
http://stackoverflow.com/questions/18784112/googletest-1-6-with-cygwin-1-7-compile-error-fileno-was-not-declared-in-this