/in/foo.cc:41:13: warning: #pragma system_header ignored outside include file
 #pragma GCC system_header
             ^~~~~~~~~~~~~
/usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status