自定义变量：
SET(opencv_dir "/mnt/d/android/OpenCV-3.4.8-android-sdk")
调用：
INCLUDE_DIRECTORIES(${opencv_dir}/sdk/native/jni/include)
SET(OpenCV_LIB ${opencv_dir}/sdk/native/libs/arm64-v8a/libopencv_java3.so)
打印出自定义变量：
语法：MESSAGE("include folder: "${include_dir})
在执行cmaek 命令的时候就会输入${include_dir}的值
判断变量用法：
判断字符，else用法，打印消息
if(address STREQUAL "arm64-v8a")
message("defined address!!!!!!!!!!")
else()
message("NOT defined address!!!!!!!!!")
endif()
option(address "hello world" ON)
message("option is ${address}")
if(address)
message("defined address!!!!!!!!!!")
else()
message("NOT defined address!!!!!!!!!")
endif()
实例：
if (${ANDROID_ABI} STREQUAL "
