Support -ios triple

Autoconf hasn’t had an update since 2014, and it doesn’t look like it will soon[1]
This updates config.{guess,sub}

It adds support for e.g. `-ios`, which allows to have targets like `aarch64-apple-ios`.

It basically does exactly what the config.guess script says:
> It is advised that you download the most up to date version of the config scripts from

The configure.ac script has been updated to relax `*-apple-darwin*` to `*-apple-*`.
Similarly the `.gitignore` and `autogen.sh` needed to be updated to respect the
newer `config.{sub,guess}`

—
[1]: http://lists.gnu.org/archive/html/autoconf/2016-07/msg00017.html

.gitignore

@@ -8,7 +8,6 @@ Makefile
 Makefile.in
 aclocal.m4
 compile
-config.*
 configure
 depcomp
 doc/libffi.info

autogen.sh

@@ -1,2 +1,2 @@
 #!/bin/sh
-exec autoreconf -v -f -i
+exec autoreconf -v -i

configure.ac

@@ -191,13 +191,13 @@ fi
 
 FFI_EXEC_TRAMPOLINE_TABLE=0
 case "$target" in
-     *arm*-apple-darwin* | aarch64-apple-darwin*)
+     *arm*-apple-* | aarch64-apple-*)
        FFI_EXEC_TRAMPOLINE_TABLE=1
        AC_DEFINE(FFI_EXEC_TRAMPOLINE_TABLE, 1,
                  [Cannot use PROT_EXEC on this target, so, we revert to
                    alternative means])
      ;;
-     *-apple-darwin* | *-*-freebsd* | *-*-kfreebsd* | *-*-openbsd* | *-pc-solaris*)
+     *-apple-* | *-*-freebsd* | *-*-kfreebsd* | *-*-openbsd* | *-pc-solaris*)
        AC_DEFINE(FFI_MMAP_EXEC_WRIT, 1,
                  [Cannot use malloc on this target, so, we revert to
                    alternative means])