autotools: Use AC_CHECK_DECL to check for getentropy

Should fix build issue on iOS.

Author: nwellnhof

configure.ac

@@ -310,13 +310,17 @@ AC_CHECK_HEADERS([glob.h])
 AM_CONDITIONAL(WITH_GLOB, test "$ac_cv_header_glob_h" = "yes")
 
 dnl Checks for library functions.
-AC_CHECK_FUNCS([getentropy gettimeofday ftime stat mmap munmap])
+AC_CHECK_FUNCS([gettimeofday ftime stat mmap munmap])
 
 AH_VERBATIM([HAVE_MUNMAP_AFTER],[/* mmap() is no good without munmap() */
 #if defined(HAVE_MMAP) && !defined(HAVE_MUNMAP)
 #  undef /**/ HAVE_MMAP
 #endif])
 
+AC_CHECK_DECL([getentropy],
+              [AC_DEFINE([HAVE_GETENTROPY], [1], [getentropy])], [],
+              [#include <sys/random.h>])
+
 dnl
 dnl Checks for inet libraries
 dnl