configure: Use libiconv on systems that lack built-in iconv

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -3726,7 +3726,7 @@ check_func  getopt
 check_func  getrusage
 check_struct "sys/time.h sys/resource.h" "struct rusage" ru_maxrss
 check_func  gettimeofday
-check_func  iconv
+check_func  iconv || check_lib2 iconv.h iconv -liconv
 check_func  inet_aton $network_extralibs
 check_func  isatty
 check_func  localtime_r