(Commented out): ugly auto-configuration of HAVE_LIBBZ2 and
HAVE_LIBLZMA definitions.

These cannot yet go in place due to lacking a way of specifying
dependent libraries, when linking into samtools etc.
Added multi-threading support for reading and writing CRAM.

Note that the samtools command only calls hts_set_threads on the
output file descriptor, so only has multi-threading writing enabled.
CRAM_OPT_VERSION now works (and only works) on an open file
descriptor, freshly opened before the call to sam_hdr_write.

This avoids the need for global variables and a requirement to process
the VERSION option at a different time to all other CRAM options.