klv: make KLV support optional via ENABLE_KLV, disabled by default

This prevents the building of the klv library and plugin, and disables KLV
support and dependency in the Pleora plugin.

gst/CMakeLists.txt

@@ -4,7 +4,11 @@ endif (OPENCV_FOUND)
 
 add_subdirectory (bayerutils)
 add_subdirectory (extractcolor)
-add_subdirectory (klv)
+
+if (ENABLE_KLV)
+  add_subdirectory (klv)
+endif ()
+
 add_subdirectory (misb)
 add_subdirectory (select)
 add_subdirectory (videoadjust)