discriptive comments on HAvE_FEATURE_AVAHI and remove unused ac_def

Change-Id: I37183561a264625505036bcc6b945a6b24f05b5a

config_host/config_features.h.in

@@ -66,6 +66,18 @@
 
 #define HAVE_FEATURE_OPENCL 0
 
+/* 
+ * Whether the OS has Avahi support, 
+ * This library is used for zeroconf service publication on the local network
+ * for the Impress remote control, so that the remote can automatically 
+ * discover the running libO instance and its IP address. 
+ * 
+ * This library should be present on most mordern linux distributions
+ * but not on "all" of them. For now, you should add --enable-avahi for 
+ * this to be taken into account. Otherwise the test of its presence 
+ * will not take place.
+ */
+
 #define HAVE_FEATURE_AVAHI 0
 
 #endif

configure.ac

@@ -11392,7 +11392,6 @@ if test "$_os" != "WINNT" -a "$_os" != "Darwin" -a "$enable_avahi" = "yes"; then
     PKG_CHECK_MODULES([AVAHI], [avahi-client >= 0.6.10],
                       [ENABLE_AVAHI="TRUE"], [ENABLE_AVAHI=""])
     if test "$ENABLE_AVAHI" = "TRUE"; then
-        AC_DEFINE(ENABLE_AVAHI)
         AC_DEFINE(HAVE_FEATURE_AVAHI)
         AVAHI_CFLAGS=$(printf '%s' "$AVAHI_CFLAGS" | sed -e "s/-I/${ISYSTEM?}/g")
     fi