Win32/MSys detect changes, README warning update

Deleted the win32 include-makefile.
Now check MSYSTEM for MINGW32: a match sets HOST to WIN32. Deleted the include for the Windows include-makefile. Deleted HOST_OVERRIDE. Error out if Win32 detected, but no POSSIBILITY of MSys detected.
Replaced all mentions of MSys in host/Makefile with WIN32.
Changed the obnoxious warning in the README file.