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

@@ -198,26 +198,31 @@ fi
 m4_pattern_allow(PKG_CONFIG_MIN_VERSION)
 PKG_PROG_PKG_CONFIG
 
-PKG_CHECK_MODULES( AVAHI,  avahi-client >= 0.6.10, 
-[ 
+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)
    AC_SUBST(AVAHI_LIBS)
    avahi=true
-],
-[
+ ],
+ [
    missing_avahi_warning=yes
    avahi=false
-])
+ ])
 
-PKG_CHECK_MODULES( AVAHI_TQT,  avahi-tqt >= 0.6.10, 
-[ 
+ PKG_CHECK_MODULES( AVAHI_TQT,  avahi-tqt >= 0.6.10,
+ [
    AC_SUBST(AVAHI_TQT_CFLAGS)
    AC_SUBST(AVAHI_TQT_LIBS)
    avahi=true
-],
-[
+ ],
+ [
    missing_avahi_warning=yes
    avahi=false
-])
+ ])
+fi
 
 AM_CONDITIONAL(COMPILE_ZEROCONF, test x$avahi = xtrue)