Require enabled wallet

Throws a build error, if configured with --disable-wallet or friends:

```
~/mastercore$ ./autogen.sh
~/mastercore$ ./configure --disable-wallet
...
checking if wallet should be enabled... no
configure: error: Master Core must be used with enabled wallet. Please do not use --without-wallet or --disable-wallet.
```

Default behavior is not changed.

Feel free to suggest another error message.