wujunze@Mac: ~/tmp
 $ cat hello.c                                                                                                                                                         [12:59:51]
-sharpinclude <stdlib.h>
int main() { exit(0); }

wujunze@Mac: ~/tmp
 $ clang hello.c                                                                                                                                                       [12:59:53]
hello.c:1:10: fatal error: "stdlib.h" file not found
-sharpinclude <stdlib.h>
         ^~~~~~~~~~
1 error generated.
FAIL

still report an error after trying to execute xcode-select install

have you met any friends to take a look at-sharp-sharp-sharp problem description