configure: Check for pstack utility in configure

Some Linux distro include pstack utility instead of gstack.
pstack and gstack have the same command line interface and
are developed by the same author.

See: http://linuxcommand.org/man_pages/pstack1.html
     http://linux.die.net/man/1/pstack

Signed-off-by: Sasha Kotchubievsky <sashakot@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>