otp_src_R15B03.tar.gz has a lot of .beam files (which are compiled .erl), and they has been just repacked into rpms. we don't have any reasons not to trust Erlang/OTP maintainers, but it would be better to do clean and rebuild .beam from the sources.

- add make clean before build. it clears .beam files bundled with
sources. our aim is to build everything from sources. (forwarded request 147242 from matwey)