network: don't enable gcrypt thread callbacks when gcrypt recent enough

From `gcrypt.h`:

> NOTE: Since Libgcrypt 1.6 the thread callbacks are not anymore used.
> However we keep it to allow for some source code compatibility if used
> in the standard way.

Otherwise, we get a deprecation warning which is turned into an error:

```
  CC       libcollectdclient_la-network_buffer.lo
../../../src/libcollectdclient/network_buffer.c:58:15: error: 'gcry_thread_cbs' is deprecated (declared at /usr/include/gcrypt.h:213) [-Werror=deprecated-declarations]
 GCRY_THREAD_OPTION_PTHREAD_IMPL;
```

Fixes: #632