configure: Check for arc4random() header definition.

Fixes Cygwin compilation.

configure

@@ -5455,7 +5455,7 @@ check_func  ${malloc_prefix}memalign            && enable memalign
 check_func  ${malloc_prefix}posix_memalign      && enable posix_memalign
 
 check_func  access
-check_func  arc4random
+check_func_headers stdlib.h arc4random
 check_func_headers time.h clock_gettime || { check_func_headers time.h clock_gettime -lrt && add_extralibs -lrt && LIBRT="-lrt"; }
 check_func  fcntl
 check_func  fork