cmake中PROJECT_SOURCE_DIR的遐思

PROJECT_SOURCE_DIR为包含PROJECT()的最近一个CMakeLists.txt文件所在的文件夹。
如以下文件结构：
test
   |
   +-----build
   |   
   +-----include   
   |
   +-----lib
   |
   +-----src
              |             
              +main.cpp
              |            
              +CMakeLists.txt   
   |   
   +-----CMakeLists.txt
test/CMakeLists.txt文件内容为

CMAKE_MINIMUM_REQUIRED(VERSION 2.8)
PROJECT(test)
ADD_SUBDIRECTORY(src)

test/src/CMakeLists.txt文件内容为

CMAKE_MINIMUM_REQUIRED(VERSION 2.8)
#PROJECT(main)

在build中执行cmake ..（两个小数点别丢了）则PROJECT_SOURCE_DIR的值为(parent_path)/test，
取消#PROJECT(main)的注释，PROJECT_SOURCE_DIR的值为(parent_path)/test/src