Make warning for native build

[DJ Delorie]

> /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" ?