simplify checking for zaptel version and location

(for linux, this is functionally equivalent to the previous
method; for FreeBSD, it re-adds inspection in $PREFIX/zaptel.h).

Please wait to regenerate the "configure" file as i have
another few pending changes to configure.ac

Not applicable to 1.4 until acinclude.m4 is also updated.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@51285 65c4cc65-6c06-0410-ace0-fbb531ad65f3