The cryptographic hash function BLAKE2 is an improved version of 
the SHA-3 finalist BLAKE. BLAKE2 offers the highest security, yet is 
fast as MD5 on 64-bit platforms and requires less RAM.

This script builds the b2sum binary.

Note: if you get 'illegal instruction' errors from b2sum at runtime,
this is due to upstream's use of -march=native optimization. Rebuild
the package with FORCE_SLACK_CFLAGS=yes to avoid this.