构建操作系统时未定义的“主要”参考

Question

我只是在制作一个简单的操作系统，但是我收到了一个错误。当我尝试构建时，我收到此错误：

joseph@joseph-linuxmint ~/Desktop/os $ make test
g++     lib/screen.cpp   -o lib/screen
/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crt1.o: In   function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
<builtin>: recipe for target 'lib/screen' failed
make: *** [lib/screen] Error 1

源代码托管在github上：https://github.com/HackingNewbie/JoeOSmakefile