https://www.cnblogs.com/gaojian/archive/2012/10/04/2711494.html对 makefile 中 eval 函数的学习体会
http://blog.sina.com.cn/s/blog_4cd5d2bb0101gsnr.html 对makefile中eval函数的学习
http://blog.csdn.net/chongyang198999/article/details/47859385 Makefile中eval函数用法
https://www.gnu.org/software/make/manual/make.html#Eval-Function