config.mk: doc ALLOCA_LIMIT, add CC=cc, and set PREFIX=/usr
config.mk: doc ALLOCA_LIMIT, add CC=cc, and set PREFIX=/usr

Signed-off-by: Mattias Andrée <3d72f033b30d63e7ff5f0b71398e9c45b66386a1@kth.se>

maandree/libkeccak,maandree/libkeccak