Why does autoconf erroneously find a function which isn't available later?

Question

On a Linux system I configured a software package (llvm) and autoconf finds the arc4random function. Here an extraction of the output during configuration:

Answer 1

According to arc4random(3), to use this function, you should include <bsd/stdlib.h> in your code and link it with -lbsd.

What this AC_CHECK_FUNCS([... arc4random ]) does is to make sure arc4random exists in your system, then define the macro named HAVE_ARC4RANDOM, but it cannot guarantee your code is using it correctly.