Minor DJGPP fixes

[Eli Zaretskii]

> - fcntl() working is a characteristic of the _host_, not the target.
> - You should determine if it works using an autoconf test.

Why didn't the original code use an Autoconf test, then?  I understand
that it was added only recently.

> - If it doesn't work, you need to open the file in blocking mode, not
>   just elide the fcntl(); otherwise the code reading the file will
>   break. 

Could you please explain what does ``open in blocking mode'' mean?
Isn't a normal default `open' good enough?  If not, why not?

> This fcntl() is there to avoid a problem that never happens in real
> life: someone does #include </dev/rmt0> and the preprocessor gets
> wedged because there's no tape.

Shouldn't people who do this get what they were asking for?  I mean,
if someone *really* wanted the preprocessor to read the tape,
shouldn't the preprocessor get stuck if there's no tape?

> Therefore, I'd be willing to dump the fcntl() call entirely and open
> the file in blocking mode on all hosts.  Does anyone else have an
> opinion?

I need to understand what a ``blocking mode'' is to form a useful
opinion.  I assume that the same code will be used to read normal
files as well.