Custom signet hint

[Sjors]

It's not possible to CPU mine on the main signet, because it's signed.

However you can create a custom signet and set the challenge to OP_TRUE (signetchallenge=51 in bitcoin.conf). This triggers an exception in the BIP where no signature is required.

The only change needed here is to add signet to GBT_RULES:

#define GBT_RULES "[\"segwit\", \"signet\"]"

See https://github.com/Sjors/cpuminer/commit/20eb197e9ce7c4396d2ffb394e4faa0281bf1c02