Update compilation instructions for MSVC++, and point out that it's
not just Unix systems which can use the GNU instructions -- it also
applies to MacOS X and Windows with either Cygwin or MinGW.
actually *use* the resulting configuration file, but it keeps autoconf
from creating an insanely long compiler command line, which can cause
problems all by itself. Eventually we can do:
#ifdef HAVE_CONFIG_H
#include "config.h"
#endif
... to get the same information.
