The Windows Makefiles provide macros for various commands.

The Windows Makefiles provide macros for various commands.
The MKDIR macro is current assigned to mkdir which causes
the cygwin/msys mkdir.exe commands to be used instead of
the cmd.exe built-in command.  Change this to use "md" instead
so that the built-in will be used by default.

In addition, permit the macros to be overridden by the top
level NTMakefile or the environment.

LICENSE MIT

Reviewed-on: http://gerrit.openafs.org/30
Verified-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>