configure: check for `res_init()` before trying other forms.

On OSX, `res_init()` is a symbol of `libsystem_info` (a dependency of
`libSystem`). So, AC_SEARCH_LIBS directly finds this symbol without an
additional library. However, once you include `resolv.h`, this becomes
a macro to `res_9_init` which is a symbol of `libresolv`. So you need
to link to `libresolv`. This is why we test first for `res_9_init` and
only after for `res_init`.

See more info about this here:
 http://blog.achernya.com/2013/03/os-x-has-silly-libsystem.html

However, on other systems, this will link to libbind while this is not
needed because `res_9_init` is a symbol of libbind while `res_init` is
correctly defined in `libc` and no odd redefinition.

So, the solution is to force `AC_SEARCH_LIBS` to include `resolv.h`
and only test for `res_init` (as a symbol or a macro, we don't
care). This is done by redefining `AC_LANG_CALL(C)` to include
`resolv.h`.