Made c4 also compile under Windows/Visual Studio.

[nubok]

I added two lines in c4.c surrounding the

#include <unistd.h>

with

#ifdef _WIN32
...
#endif

to make c4 compile and run under Windows/Visual Studio, since unistd.h does not exist under Windows/Visual Studio (it is a very UNIX-specific header). Note that despite looking otherwise the macro _WIN32 is defined both when the conpilation target is 32 bit and 64 bit (cf. https://msdn.microsoft.com/en-us/library/b0084kay.aspx for reference) - thus adding this line does not add a future liability to consider when it should be planned to port c4 to 64 bit.

I also added instructions to README.md in similar spirit to the existing guidelines how to do the same "experiments" in a suitable "Command Prompt for VS 2017".