fix superbuild cmake < 3.17 (#193)

* CPM_INDENT in the global scope as suggest

* Fix FetchContent failure with cmake < 3.17

* format

cmake/CPM.cmake

@@ -41,6 +41,9 @@ It is recommended to upgrade CPM to the most recent version. \
 See https://github.com/TheLartians/CPM.cmake for more information."
       )
     endif()
+    if(${CMAKE_VERSION} VERSION_LESS "3.17.0")
+      include(FetchContent)
+    endif()
     return()
   endif()
 
@@ -133,7 +136,10 @@ include(CMakeParseArguments)
 
 # Initialize logging prefix
 if(NOT CPM_INDENT)
-  set(CPM_INDENT "CPM:")
+  set(CPM_INDENT
+      "CPM:"
+      CACHE INTERNAL ""
+  )
 endif()
 
 function(cpm_find_package NAME VERSION)