Updated BOB_SOURCE_DIR handling

FindBob.cmake

@@ -197,9 +197,8 @@ if (NOT Bob_FIND_COMPONENTS)
   message(FATAL_ERROR "Please specify the bob packages that you want to search for as COMPONENTS")
 endif()
 
-if (NOT BOB_SOURCE_DIR)
-  set(BOB_SOURCE_DIR ${CMAKE_SOURCE_DIR})
-endif()
+# set Bob source directory, if not given yet (i.e., no FORCE option here)
+set(BOB_SOURCE_DIR ${CMAKE_SOURCE_DIR} CACHE STRING "Source directory of Bob packages")
 
 # set BOB_PREFIX_PATH as CMake variable when given in as environment variable
 if (NOT "$ENV{BOB_PREFIX_PATH}" STREQUAL "")