Windows: allow input and output flags to end with a space, or not

With a number of tools, especially those coming with Visual Studio,
some command options are separated from their argument with a space,
others with a space.  Since we parametrise them, we can't know
beforehand which it will be, so we must allow the input and output
options to have either.

However, spaces at the end of nmake macro values are trimmed, so allow
spaces to exist by adding a reference to an undefined macro at the end.

Reviewed-by: Andy Polyakov <appro@openssl.org>