printing: List Avahi printers

Show printers advertised by avahi on local network. CUPS
backend now looks for _ipps._tcp and _ipp._tcp services
offered by avahi. If it finds such a service (printer)
it requests its attributes through IPP_GET_PRINTER_ATTRIBUTES
ipp request and adds it to the list of printers. Such printer
behaves like a remote printer then.
If an avahi printer is a default printer then it is considered
default by the backend only if there is no local or remote
default printer.
This functionality is enabled when building Gtk+ with CUPS 1.6
or later because it replaces browsing protocol removed in CUPS 1.6.

https://bugzilla.gnome.org/show_bug.cgi?id=688956

configure.ac

@@ -1421,7 +1421,7 @@ else
 	    $CUPS_API_MAJOR -eq 1 -a $CUPS_API_MINOR -ge 6; then
       AC_DEFINE(HAVE_CUPS_API_1_6, 1,
                 [Define to 1 if CUPS 1.6 API is available])
-
+      have_cups_api_1_6=yes
     fi
 
     AC_SUBST(CUPS_API_MAJOR)
@@ -1452,6 +1452,45 @@ else
   fi
 fi
 
+# Check for Avahi flags
+AC_ARG_ENABLE(avahi-browsing,
+              [AS_HELP_STRING([--disable-avahi-browsing],
+                              [disable avahi browsing of printers])],,
+              [enable_avahi_browsing=auto])
+
+have_avahi_browsing=no
+if test "x$enable_avahi_browsing" != "xno" -a \
+        "x$have_cups_api_1_6" = "xyes"; then
+  AC_MSG_CHECKING([avahi-gobject])
+
+  if $PKG_CONFIG --exists avahi-gobject ; then
+    AVAHI_CFLAGS=`$PKG_CONFIG --cflags avahi-gobject`
+    AC_SUBST(AVAHI_CFLAGS)
+    AVAHI_LIBS=`$PKG_CONFIG --libs avahi-gobject`
+    AC_SUBST(AVAHI_LIBS)
+    have_avahi_gobject=yes
+    AC_MSG_RESULT([yes])
+  else
+    AC_MSG_RESULT([no])
+  fi
+
+  if test "x$have_avahi_gobject" = "xyes" ; then
+    AC_DEFINE([HAVE_AVAHI_BROWSING], [1], [Define to 1 if avahi-gobject available])
+    have_avahi_browsing=yes
+  else
+    AC_MSG_ERROR([
+*** avahi-gobject not found. avahi-gobject is required to build GTK+ with support
+*** for avahi browsed printers when using CUPS 1.6.
+])
+  fi
+else
+  if test "x$enable_avahi_browsing" = "xyes"; then
+    AC_MSG_ERROR([
+*** Avahi browsing support requested but CUPS 1.6 not found.
+])
+  fi
+fi
+
 # Checks to see if we should compile with PAPI backend for GTK+
 #
 
@@ -1828,6 +1867,7 @@ echo "        Dynamic modules:      $build_dynamic_modules"
 echo "        Included immodules:   $included_immodules"
 echo "        PackageKit support:   $build_packagekit"
 echo "        colord support:       $have_colord"
+echo "        Avahi browsing:       $have_avahi_browsing"
 echo "        Introspection:        $found_introspection"
 echo "        Debugging:            $enable_debug"
 echo "        Documentation:        $enable_gtk_doc"