only build growl bacend if it is checked out

src/plugins/backends/growl/CMakeLists.txt

@@ -1,4 +1,5 @@
 if( WITH_GROWL_BACKEND )
+    if(EXISTS ${CMAKE_SOURCE_DIR}/thirdparty/gntp-send/)
         message( STATUS "Adding libgrowl backend" )
 
         find_package(Threads REQUIRED)
@@ -19,5 +20,8 @@ if( WITH_GROWL_BACKEND )
         endif()
 
         install(TARGETS libsnore_backend_growl ${SNORE_PLUGIN_INSTALL_PATH})
+    else()
+        message( WARNING "To build the Growl Backend please checkout the submodule" )
+    endif()
 
 endif( WITH_GROWL_BACKEND )