configure: check and define if_nametoindex() on OS/2

This fixes the following compilation breakge on OS/2.

-----
  CC       network/udp.lo
network/udp.c: In function 'net_SetMcastOut':
network/udp.c:259:17: error: implicit declaration of function 'if_nametoindex'; did you mean 'if_nameindex'? [-Werror=implicit-function-declaration]
     int scope = if_nametoindex (iface);
                 ^~~~~~~~~~~~~~
                 if_nameindex
-----

Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>

configure.ac

@@ -290,6 +290,7 @@ case "${host_os}" in
     LDFLAGS="${LDFLAGS} -Zomf -Zbin-files -Zargs-wild -Zhigh-mem"
     AC_LIBOBJ([freeaddrinfo])
     AC_LIBOBJ([gai_strerror])
+    AC_CHECK_FUNCS([if_nametoindex])
     ;;
   *nacl*)
     SYS=nacl

include/vlc_fixups.h

@@ -602,6 +602,10 @@ static const struct in6_addr in6addr_any =
 # ifndef EPROTO
 #  define EPROTO (ELAST + 1)
 # endif
+
+# ifndef HAVE_IF_NAMETOINDEX
+#  define if_nametoindex(name)  atoi(name)
+# endif
 #endif
 
 /* math.h */