Currently, each of the mentioned eclasses has its own *_BUILD_DIR.
Therefore, if someone needs to provide a custom BUILD_DIR, he needs to
set it explicitly for the eclass. This is fine for ebuilds but not
really neat for eclasses.
The idea is simple: use a common BUILD_DIR instead. For compatibility,
export AUTOTOOLS_ & CMAKE_BUILD_DIR as well but prefer the former one.
For most of the ebuilds this won't make a difference.
I'm attaching an example conversion of pygobject where this would
benefit. The idea is quite simple; ebuild calls
python_foreach_impl autotools-utils_src_...
python_foreach_impl() sets BUILD_DIR for each implementation,
autotools-utils phases use that and everything simply works!
