Add md5 through extras/libmd5 taken from HM with BSD license. It's
implemented as a generic strategy using the same interface as checksum,
so we can write a SIMD version if it seems necessary.
Prevents a conflict with config.h and src/config.h so that the config.h
generated by configure is included in global.h. Fixes problems with
large input files on 32-bit systems.
Option -mavx2 was omitted when compiling AVX2 strategies. This commit
moves strategies to convenience libraries so that their compilation
flags can be easily set and adds -mavx2 to CFLAGS of the AVX2 library.