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