Fix support for parallel building with make (-j).

Previously make -j <N> would cause a race between doing cleanup of
certain files (defaults.h, menuselect, ...) and creating them anew.
Add a new target that depends on cleanup only and has a submake doing
the rest as command string. This way the cleanup goes first.

(closes issue ASTERISK-18751)
Tested by: Jeremy Kister
Reviewed by: Paul Belanger
Review: https://reviewboard.asterisk.org/r/1660


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@351360 65c4cc65-6c06-0410-ace0-fbb531ad65f3