linux知识点——CMakeLists.txt中add

参考：https://blog.csdn.net/qq_35699473/article/details/115837708

message，编译时打印变量的值。

-- The C compiler identification is GNU 7.5.0
-- The CXX compiler identification is GNU 7.5.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
ON
itisON
-- Configuring done
-- Generating done
-- Build files have been written to: /home/wuhd/wuhd/code/mydemo/add_definitions/build

CMake中的option用于控制编译流程，相当于C语言中的宏条件编译。

options基本格式如下：

option( "" [value])

variable：定义选项名称
help_text：说明选项的含义
value:定义选项默认状态，一般是OFF或者ON，除去ON之外，其他所有值都为认为是OFF

 option(TEST_IT_CMAKE "test" ON)  

这里test是注释说明的作用，TEST_IT_CMAKE这里是一个变量，定义为ON;
通过add_definitions将其转换为#define TEST_IT_CMAKE ON