Make warning for native build

[Nathanael Nerode]

>> /me thinks BUILD_SUBDIR != TARGET_SUBDIR should always hold, so that
>> we use the newly build gcc to build libiberty for the target, even if
>> target == host == build.  An easy way to accomplish this is to prefix
>> BUILD_SUBDIR with build-, always or when build_alias = target_alias.
>
>Good idea.  I don't think we need to worry about host == build because
>they'd both be built with the system compiler anyway.  But if
>build==host it should use ./libiberty, not ./$build/libiberty.  The
>only time we would even have this conflict is if build != host and
>build == target, because that's the only time that build has a
>./$build subdirectory that matches ./$target.
>
>So if build==target and build!=host, then builddir="build-$build" ?

I like. :-)  But how about making builddir="build-$build" always?
If build=host, those rules aren't activated anyway.  When build!=host 
and host==target, I don't see why we shouldn't give it this name; it's 
no worse than the existing name.  This allows a greater degree of 
consistency.

--Nathanael