Some notes on ROOT:
* The idea behind ROOT is that one can build a system with ROOT=/somewhere and
then chroot into it or tar up /somewhere as a system image. It is not designed
to allow the user to run /somewhere/usr/bin/foo.
* ROOT can't be used correctly in src_*, since ROOT might be different when
merging a binary package. For example, a binary package might be built with
ROOT=/ and then installed onto a system using ROOT=/somewhere.
* ROOT isn't used for library dependencies. When building, libraries, headers
etc. on / have to be used. Cross-compiling etc is done by configuring the
toolchain correctly, not by hacking ebuilds in a way that breaks binary
See also the discussion on bug #167285 for more info.
The 3rd part of this statement is incorrect. ROOT is exactly used for lib deps and includes.