Honor DESTDIR in w32api and mingw

[Ralf Wildenhues]

* Dave Korn wrote on Sun, Oct 18, 2009 at 06:08:32PM CEST:
> 
>   Well, I can think of a possible counter-proposal: how about a patch that
> adds DESTDIR in the normal manner, but only on platforms that support DESTDIR
> correctly?  This could be done by testing the --host setting in the Makefile
> and either warning, erroring, or just silently overriding the definition of
> DESTDIR to empty on platforms that don't can't or won't honour it.  There
> shouldn't be anything particularly controversial about the concept of using a
> feature on some platforms where it's implemented and not on others where it isn't.

Except, of course, a string '$(DESTDIR)' that expands to an empty
string, doesn't really impact any platform on which DESTDIR support
doesn't work.

IOW, it's purely a "when users look at our makefiles, they might think
it could work" thing.

Cheers,
Ralf