configure.ac: add check for popt library

The popt library is used by babeltrace but isn't checked in
configure.ac, so the compilation fails due to popt.h being missing if
popt is not already installed on the system.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>