NMake Makefiles: Use 'md', not 'mkdir'
NMake Makefiles: Use 'md', not 'mkdir'

This avoids confusion with the unixy ones supplied by Cygwin or MSYS64,
since we could be using tools from there to enable building from GIT
checkouts.

GNOME/atkmm,GNOME/atkmm,GNOME/atkmm,GNOME/atkmm