Enable Avahi support by default if possible. Re #224

This eases building of the escl backend and aligns configure time
behaviour with other --with-* options.

NEWS

@@ -11,6 +11,10 @@
 ### Build
 
 - removes the `--with-api-spec` option from `configure`
+- replaces the `--enable-avahi` option with an `--with-avahi` that
+  defaults to enabling if possible.  If the option is given and the
+  required support is not available, `configure` will exit with an
+  error.
 
 
 ## New with 1.0.29 (released 2020-02-02)

acinclude.m4

@@ -613,7 +613,7 @@ for be in ${BACKENDS}; do
     ;;
 
     escl)
-    if test "x${enable_avahi}" != "xyes"; then
+    if test "x${with_avahi}" != "xyes"; then
       echo "*** $be backend requires AVAHI library - $DISABLE_MSG"
       backend_supported="no"
     fi

configure.ac

@@ -132,14 +132,23 @@ if test "$with_v4l" != "no" ; then
   PKG_CHECK_MODULES(LIBV4L, libv4l1, have_libv4l1=yes, have_libv4l1=no)
 fi
 
-AC_ARG_ENABLE(avahi,
-	AS_HELP_STRING([--enable-avahi], [enable Avahi support for saned and the net backend]),
-	[enable_avahi=$enableval], [enable_avahi=no])
-
-if test "$enable_avahi" = "yes"; then
-  PKG_CHECK_MODULES(AVAHI, [ avahi-client >= 0.6.24 ],
-                    [AC_DEFINE(WITH_AVAHI, 1, [define if Avahi support is enabled for saned and the net backend])], enable_avahi=no)
-fi
+AC_ARG_WITH(avahi,
+  AS_HELP_STRING([--with-avahi],
+    [enable Avahi support @<:@default=check@:>@]),
+  [],
+  [with_avahi=check])
+AC_DEFINE(WITH_AVAHI,
+  [0], [Define to 1 if Avahi support is available])
+AS_IF([test xno != "x$with_avahi"],
+  [PKG_CHECK_MODULES(AVAHI, [avahi-client >= 0.6.24],
+    [AC_DEFINE([WITH_AVAHI], [1])
+     with_avahi=yes
+    ],
+    [AS_IF([test xcheck != "x$with_avahi"],
+       [AC_MSG_ERROR([Avahi support requested but not found])])
+     with_avahi=no
+    ])
+  ])
 AM_CONDITIONAL([have_libavahi], [test x != "x$AVAHI_LIBS"])
 
 dnl check sane to make sure we don't have two installations
@@ -817,7 +826,7 @@ else
   echo "Build saned:   no"
 fi
 echo "IPv6 support:  `eval eval echo ${ipv6}`"
-echo "Avahi support: `eval eval echo ${enable_avahi}`"
+echo "Avahi support: `eval eval echo ${with_avahi}`"
 echo "cURL support:  `eval eval echo ${with_libcurl}`"
 echo "SNMP support:  `eval eval echo ${with_snmp}`"
 echo "-> The following backends will be built:"