build: Do not use `mkdir -p` on Windows

When the build is performed using `cmd.exe` as shell, the `mkdir`
command does not support the `-p` flag. The ability to create multiple
netsted folder is not used, hence it can be easily replaced by only
creating the directory if it does not exist.

This makes the build work on the `cmd.exe` shell, except for the
`clean` targets.

Signed-off-by: Andrea Canciani <ranma42@gmail.com>
Acked-by: Oded Gabbay <oded.gabbay@gmail.com>