change the check for zaptel to only check for linux/zaptel.h on Linux, and

change the check for zaptel to only check for linux/zaptel.h on Linux, and
zaptel.h otherwise

If you did ./configure --with-tonezone=/usr/local/ on FreeBSD, it would run
the first macro which would not find it, but it wouldn't make it to the second
macro because the first check saw that --with-tonezone was specified and would
exit with error

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@25989 65c4cc65-6c06-0410-ace0-fbb531ad65f3