Update AX_CHECK_SIGN for autoconf 2.71

Update AX_CHECK_SIGN for autoconf 2.71

When running autoreconf with ax_check_sign.m4, autoconf 2.71 complains
about the use of `AX_TRY_COMPILE`:

```
libtoolize: copying file 'm4/lt~obsolete.m4'
configure.ac:29: warning: The macro `AC_TRY_COMPILE' is obsolete.
configure.ac:29: You should run autoupdate.
./lib/autoconf/general.m4:2847: AC_TRY_COMPILE is expanded from...
lib/m4sugar/m4sh.m4:692: _AS_IF_ELSE is expanded from...
lib/m4sugar/m4sh.m4:699: AS_IF is expanded from...
./lib/autoconf/general.m4:2249: AC_CACHE_VAL is expanded from...
./lib/autoconf/general.m4:2270: AC_CACHE_CHECK is expanded from...
m4/ax_check_sign.m4:41: AX_CHECK_SIGN is expanded from...
m4/pdns_check_time_t.m4:1: PDNS_CHECK_TIME_T is expanded from...
configure.ac:29: the top level
```

This updates the macro to use `AC_COMPILE_IFELSE`, which has been present in
autoconf since at least version 2.60, released in 26 Jun 2006 (I did not
check any older releases).

This patch has been [submitted](http://savannah.gnu.org/patch/index.php?10071) [upstream](https://github.com/autoconf-archive/autoconf-archive/pull/225).

(cherry picked from commit 5e180ac61a7e73dc7681a2d34e6faf6908f14f6d)