Without this flag, mingw-w64 uses the MS implementations of snpintf and the likes.
This is not really a problem since they work with the the fixes provided for msvc,
but due to the way mingw-w64's stdio.h is structured we cannot get it to use the fixes.
By default mingw outputs 3 digits instead of the standard 2 so all float
tests using printf fail. Using setenv at the start of the program fixes this.
To accomodate calling platform specific initialization a
MICROPY_MAIN_INIT_FUNC macro is used which is called in mp_init()
The mingw port used _fullpath() until now, but the behaviour is not exactly
the same as realpath()'s on unix; major difference being that it doesn't
return an error for non-existing files, which would bypass main's error
checking and bail out without any error message.
Also realpath() will return forward slashes only since main() relies on that.
The autogenerated header files have been moved about, and an extra
include dir has been added, which means you can give a custom
BUILD=newbuilddir option to make, and everything "just works"
Also tidied up the way the different Makefiles build their include-
directory flags
