Add an explicit --disable-avahi to the configuration options.

Previous defaults remain --- the option must be explicitly
defined to be used. Thanks to E. Liddell.

configure.in.in

@@ -198,6 +198,10 @@ fi
 m4_pattern_allow(PKG_CONFIG_MIN_VERSION)
 PKG_PROG_PKG_CONFIG
 
+AC_ARG_WITH([avahi], AS_HELP_STRING([--without-avahi], [Build without avahi/ZeroConf support (default: test)]))
+
+avahi=false
+if test "x$with_avahi" != "xno"; then
  PKG_CHECK_MODULES( AVAHI,  avahi-client >= 0.6.10,
  [
    AC_SUBST(AVAHI_CFLAGS)
@@ -219,5 +223,6 @@ PKG_CHECK_MODULES( AVAHI_TQT,  avahi-tqt >= 0.6.10,
    missing_avahi_warning=yes
    avahi=false
  ])
+fi
 
 AM_CONDITIONAL(COMPILE_ZEROCONF, test x$avahi = xtrue)