Create a small wrapper for setenv() for portability to Windows

Some of our tests need to set environment variables.  We need a wrapper for `::setenv()` as this is a Unix specific call.  h/t @mbrukman that we can use this trick from abseil:

```C++
#ifdef WIN32
#include <windows.h>
#endif

#ifdef WIN32
      SetEnvironmentVariable(...);
#else
      setenv(...);
#endif
``` 