Properly detect launchd

Asterisk was a little too pro-active in claiming that it found launchd. On
systems without launchd - such as FreeBSD - this resulted in certain items
in Asterisk that conflict with launchd to not be selectable, such as
res_timing_kqueue.

(closes issue ASTERISK-20749)
Reported by: Oleg Baranov
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Merged revisions 381847 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 381848 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@381869 65c4cc65-6c06-0410-ace0-fbb531ad65f3

configure.ac

@@ -2296,7 +2296,9 @@ PBX_LAUNCHD=0
 if test "${cross_compiling}" = "no";
 then
   AC_CHECK_FILE(/sbin/launchd, AC_DEFINE([HAVE_SBIN_LAUNCHD], 1, [Define to 1 if your system has /sbin/launchd.]))
-  PBX_LAUNCHD=1
+  if test "${HAVE_SBIN_LAUNCHD}" = 1; then
+    PBX_LAUNCHD=1
+  fi
 fi
 AC_SUBST(PBX_LAUNCHD)