configure.in: syscall is deprecated on macOS

* configure.in: syscall is no longer supported on macOS since
  10.12.  [ruby-core:80300] [Bug #13361]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58084 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.in

@@ -1100,6 +1100,12 @@ AS_CASE(["$target_os"],
 		    ],
 		    [macosx_10_5=yes], [macosx_10_5=no])
 		AC_MSG_RESULT($macosx_10_5)
+		AS_IF([test "${target_os@%:@darwin}" -ge 16], [
+		    ac_cv_func___syscall=no
+		    ac_cv_func_syscall=no
+		    ac_cv_header_sys_syscall_h=no
+		    ac_cv_header_syscall_h=no
+		])
 		if test $macosx_10_5 = yes; then
 		    ac_cv_func_getcontext=no
 		    ac_cv_func_setcontext=no