Why does autoconf mistakenly find a function that is not available later?

Question

On Linux, I configured the software package (llvm) and autoconf finds the function arc4random. Here's the extraction of output during configuration:

checking for strerror... yes
checking for strerror_r... yes
checking for setenv... yes
checking for arc4random... yes
checking for strtoll... yes
checking for strtoq... yes
checking for sysconf... yes

The configuration is going fine. Later, when creating the package, I get an error message specified by an unspecified specifier arc4random:

[removed]/lib/Support/Unix/Process.inc:368:10: error: use of undeclared identifier
      'arc4random'
  return arc4random();
         ^

Here is the location indicated:

367 #if defined(HAVE_ARC4RANDOM)
368   return arc4random();
369 #else
370   static int x = (::srand(GetRandomNumberSeed()), 0);
371   (void)x;
372   return ::rand();
373 #endif

It is properly guarded, here is configure.acpart:

AC_CHECK_FUNCS([strerror strerror_r setenv arc4random ])

Everything seems to be in order. I am wondering why the configure process has detected that the function will be available.

autoconf (GNU Autoconf) 2.63

Here is an excerpt from config.log:

configure --prefix=[removed] --host=powerpc64-bgq-linux --disable-terminfo --disable-zlib --enable-targets=powerpc CXX=bgclang++ CXXFLAGS=-O3 -fPIC CC=bgclang CFLAGS=-O3 -fPIC LDFLAGS=-shared

Answer 1

arc4random(3), , <bsd/stdlib.h> -lbsd.

AC_CHECK_FUNCS([... arc4random ]), , arc4random, HAVE_ARC4RANDOM, , .