Report OpenSSL build date too.

Adjust `-d -bV` output for OpenSSL to include library build date.

Some OS packagers have backported heartbleed security fixes without
changing anything in the reported version number.  The closest we can
get to a reassuring sign for administrators is to report the OpenSSL
library build date, as picked by the library which Exim is using at run
time.

```
Library version: OpenSSL: Compile: OpenSSL 1.0.1g 7 Apr 2014
                          Runtime: OpenSSL 1.0.1g 7 Apr 2014
                                 : built on: Mon Apr  7 15:08:30 PDT 2014
```

For comparison, the version information for OpenSSL on Ubuntu (where
Exim is by default built with GnuTLS, but this provides for context for
comparison):

```
$ openssl version -v -b
OpenSSL 1.0.1 14 Mar 2012
built on: Mon Apr  7 20:33:29 UTC 2014
```

GnuTLS: the closest I can find to a runtime value is the call we are
already making; if an OS vendor patches GnuTLS without changing the
version which would be returned by `gnutls_check_version(NULL)` then the
sysadmin is SOL and will have to explore library linkages more
carefully.