remove relative paths and use ASTTOPDIR instead.

remove relative paths and use ASTTOPDIR instead.

Give a default value to ASTTOPDIR if unset so we can at least
do a 'make clean' without too much trouble.

The proper fix, however, is to partition the top level
Makefile in a 'setup' and a 'main' part, in a way that the
'setup' part can be included from subdirs' Makefiles and
allow targets to be built without going through the
top level Makefile.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@92104 65c4cc65-6c06-0410-ace0-fbb531ad65f3