fio: Add support for auto detect dev-dax and libpmemblk engines

Added support code in configure to detect whether libpmem and libpmemblk
are installed. dev-dax engine is enabled if libpmem is installed.
pmemblk engine is installed if libpmem and libpmemblk are installed.
A --disable-pmem flag is added to explicitly disable the libpmem based
engines even if the libs are detected.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>