Document LRC's DRING_XML_INTERFACES_DIR cmake flag

Change-Id: I7b18d4925c997ebd7440ca0a0ba2ce17858664eb
Tuleap: #688

docs/source/compiling_and_installing/lrc.rst

@@ -22,6 +22,7 @@ The following options are often usefull to append to the cmake line:
 
     -DRING_BUILD_DIR=<daemon install location>
     -DCMAKE_INSTALL_PREFIX=<install location>
+    -DRING_XML_INTERFACES_DIR=<daemon dbus interface definitions directory>
     -DCMAKE_BUILD_TYPE=<Debug to compile with debug symbols>
     -DENABLE_VIDEO=<False to disable video support>