adding support for SAM/BAM output using htslib.  Please turn on within

the Makefile.
Htslib installs into e.g. /usr/include/htslib/sam.h; the intention is for user code to `#include <htslib/sam.h>` and thus for your makefile to need only `-I ../htslib`.  (Yes, this means that to use htslib's kstring you need to conditionalize that inclusion on `USE_HTSLIB` too.)