Use externally defined paths for demosaic packs and rawspeed before falling back to defaults.

--- LibRaw-0.16.0-Alpha2/CMakeLists.txt
+++ LibRaw-0.16.0-Alpha2/CMakeLists.txt
@@ -81,9 +81,15 @@
 SET(RAWSPEED_RPATH           "RawSpeed"           CACHE STRING 
     "Relavive path to extra RawSpeed codec     (default=RawSpeed)")
 
-SET(DEMOSAIC_PACK_GPL2_PATH "${CMAKE_CURRENT_SOURCE_DIR}/${DEMOSAIC_PACK_GPL2_RPATH}")
-SET(DEMOSAIC_PACK_GPL3_PATH "${CMAKE_CURRENT_SOURCE_DIR}/${DEMOSAIC_PACK_GPL3_RPATH}")
-SET(RAWSPEED_PATH           "${CMAKE_CURRENT_SOURCE_DIR}/${RAWSPEED_RPATH}")
+IF(NOT DEMOSAIC_PACK_GPL2_PATH)
+    SET(DEMOSAIC_PACK_GPL2_PATH "${CMAKE_CURRENT_SOURCE_DIR}/${DEMOSAIC_PACK_GPL2_RPATH}")
+ENDIF()
+IF(NOT DEMOSAIC_PACK_GPL3_PATH)
+    SET(DEMOSAIC_PACK_GPL3_PATH "${CMAKE_CURRENT_SOURCE_DIR}/${DEMOSAIC_PACK_GPL3_RPATH}")
+ENDIF()
+IF(NOT RAWSPEED_PATH)
+    SET(RAWSPEED_PATH           "${CMAKE_CURRENT_SOURCE_DIR}/${RAWSPEED_RPATH}")
+ENDIF()
 
 # ==================================================================================================
 # General definitions rules