Making cache default setting more robust.

Author: sean-parent

CMakeLists.txt

@@ -4,8 +4,9 @@ cmake_minimum_required(VERSION 3.20)
 project(enum-ops)
 
 # Setup cpp-library infrastructure
-if(PROJECT_IS_TOP_LEVEL)
-    set(CPM_SOURCE_CACHE ${CMAKE_SOURCE_DIR}/.cache/cpm CACHE PATH "CPM cache")
+if(PROJECT_IS_TOP_LEVEL AND NOT CPM_SOURCE_CACHE AND NOT DEFINED ENV{CPM_SOURCE_CACHE})
+    set(CPM_SOURCE_CACHE "${CMAKE_SOURCE_DIR}/.cache/cpm" CACHE PATH "CPM source cache")
+    message(STATUS "Setting cpm cache dir to: ${CPM_SOURCE_CACHE}")
 endif()
 include(cmake/CPM.cmake)