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.