config: libintl/libiconv for gettext() detection

Detect in autoconf whether `-lintl` and possibly `-liconv` are necessary
for translation functions like `gettext()`.

The actual autoconf code is just:

```
AM_ICONV
AM_GNU_GETTEXT([external])
LIBS="$LIBS $LTLIBINTL $LTLIBICONV"
```

References:

https://www.gnu.org/software/gettext/manual/html_node/AM_005fGNU_005fGETTEXT.html
https://www.gnu.org/software/gettext/manual/html_node/AM_005fICONV.html

The reason to check for `libiconv` and add it separately is that this is
sometimes necessary if users are linking statically.

The `config/*.m4` files were added by running `gettextize` and removing
everything else.

The empty file `config/config.rpath` is necessary to avoid an error with
some versions of autotools, see:

http://ramblingfoo.blogspot.com/2007/07/required-file-configrpath-not-found.html

The `config.rpath` copied by `gettextize` does not currently work, there
is some kind of missing interaction with `libtool` and it tries to apply
`libtool` flags to the compiler.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Rafael Kitover <rkitover@gmail.com>
Closes #8554

config/user.m4

@@ -2,6 +2,7 @@ dnl #
 dnl # Default ZFS user configuration
 dnl #
 AC_DEFUN([ZFS_AC_CONFIG_USER], [
+	ZFS_AC_CONFIG_USER_GETTEXT
 	ZFS_AC_CONFIG_USER_MOUNT_HELPER
 	ZFS_AC_CONFIG_USER_UDEV
 	ZFS_AC_CONFIG_USER_SYSTEMD