Compiling as root has some hazards because of how complicated configure and make scripts can
be. It isn't that there might be a hidden trojan in the code (after all you're going to run
make install anyway right), but that the build scripts might accidentally do bad things.
For example, I once saw a makefile try to delete and rebuild some system library because it
was listed as a dependency that got make's automatic target rules all excited. It only tried
that on certain configurations of machine, never the developer's so he never saw the problem.