ragel: disable unused variable warnings for generated source

FindRAGEL.cmake

@@ -96,6 +96,16 @@ ${RAGEL_version_error}")
           "[RAGEL][${Name}] Compiling state machine with Ragel ${RAGEL_VERSION}"
       WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
 
+    # Disable some warnings the user may have set which ragel will trigger.
+    check_cxx_compiler_flag ("-Wno-unused-const-variable" __no_unused_const_variable)
+    if (__no_unused_const_variable)
+        set_source_files_properties (
+            ${Output}
+        PROPERTIES
+            COMPILE_FLAGS -Wno-unused-const-variable
+        )
+    endif()
+
     set(RAGEL_${Name}_DEFINED       TRUE)
     set(RAGEL_${Name}_OUTPUTS       ${Output})
     set(RAGEL_${Name}_INPUT         ${Input})