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.txttest/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