ENH: Allow to overwrite CPACK_BINARY_DIR

Modules/CPack.cmake

@@ -67,7 +67,7 @@ IF(NOT CPACK_GENERATOR)
 ENDIF(NOT CPACK_GENERATOR)
 
 # Set some other variables
-SET(CPACK_BINARY_DIR "${CMAKE_BINARY_DIR}")
+cpack_set_if_not_set(CPACK_BINARY_DIR "${CMAKE_BINARY_DIR}")
 
 SET(_CPACK_UNUSED_VARIABLES_)
 GET_CMAKE_PROPERTY(res VARIABLES)