BSD detection

There is this piece of code in headers.h:

#ifdef __WXMAC_OSX__
#define __WXMAC__ 1
#define __WXOSX__ 1
#define __BSD__ 1
#endif
#endif


In my testing it's not caught on FreeBSD 8.1

I propose it be changed to:

#if (defined(__unix__) || defined(unix)) && !defined(USG)
#include <sys/param.h>
#endif


And then checks for BSD should be
#ifdef BSD
This is the recommended way of detecting BSD in the FreeBSD Porter's Handbook.

This change is already done in my CMake tree.

Don't know if Mac OS X is technically a BSD and I don't have access to a Mac, maybe someone else may test it.
If there are other Mac (and not BSD) specialties, then we'd need to check both for __WXMAC_OSX__ and BSD or detect these in some other way altogether.