Allow mining RPCs with --disable-wallet

The following mining-related RPC calls don't use the wallet:

- getnetworkhashps
- getmininginfo
- getblocktemplate
- submitblock

Enable them when compiling with --disable-wallet.

doc/build-unix.md

@@ -187,3 +187,7 @@ disable-wallet mode with:
     ./configure --disable-wallet
 
 In this case there is no dependency on Berkeley DB 4.8.
+
+Mining is also possible in disable-wallet mode, but only using the `getblocktemplate` RPC
+call not `getwork`.
+