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 <[email protected]>
Reviewed-by: Richard Laager <[email protected]>
Signed-off-by: Rafael Kitover <[email protected]>
Closes #8554 

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