configure, README: introduce --enable-external-plugins

As the README chunk says, disabled by default, since they rely on
internal API/ABI and can break at any point.

Instead everyone affected should work and upstream their plugin into the
bluez project.

README

@@ -249,6 +249,19 @@ For a working system, certain configuration options need to be enabled:
 		systems. The behavior of the deprecated tools may be unstable
 		or simply don't work anymore.
 
+	--enable-external-plugins
+
+		Enable support for external plugins
+
+		By default external plugins for bluetoothd and obexd are not
+		supported and thus disabled.
+
+		External plugins require access to internal, undocumented and
+		unversioned API in said daemons. As such they can break at any
+		time. If you have such plugins, enable this option and work
+		actively with the community to make said plugin part of the
+		upstream bluez project.
+
 	--enable-nfc
 
 		This option enable NFC pairing support.

configure.ac

@@ -364,6 +364,16 @@ AC_ARG_ENABLE(deprecated, AS_HELP_STRING([--enable-deprecated],
 					[enable_deprecated=${enableval}])
 AM_CONDITIONAL(DEPRECATED, test "${enable_deprecated}" = "yes")
 
+AC_ARG_ENABLE(external-plugsin, AS_HELP_STRING([--enable-external-plugins],
+			[enable support for external plugins]),
+					[enable_external_plugins=${enableval}])
+AM_CONDITIONAL(EXTERNAL_PLUGINS, test "${enable_external_plugins}" = "yes")
+if (test "${enable_external_plugins}" = "yes"); then
+	AC_DEFINE(EXTERNAL_PLUGINS, 1, [Define if external plugin support is required])
+else
+	AC_DEFINE(EXTERNAL_PLUGINS, 0, [Define if external plugin support is required])
+fi
+
 AC_ARG_ENABLE(sixaxis, AS_HELP_STRING([--enable-sixaxis],
 		[enable sixaxis plugin]), [enable_sixaxis=${enableval}])
 AM_CONDITIONAL(SIXAXIS, test "${enable_sixaxis}" = "yes" &&