Dependencies:
	Jansson 2.0 with 'long long' support

Example dependencies:
	Jansson 2.1 (to read JSON from stdin)
	libgcrypt (for SHA256)

For usage, check out example.c
Note that you must assign blkmk_sha256_impl to a function pointer:
	bool mysha256(void *hash_out, const void *data, size_t datasz)
hash_out must be able to overlap with data!

Also note that you should NOT roll ntime for data retrieved; while it will
probably work, there is no guarantee it won't fall outside the maxtime limits
for the blocktemplate. It is usually best to simply get more data as often
as it is needed.