package/poppler: fix build with cmake < 3.10

package/poppler: fix build with cmake < 3.10

poppler requires threads, and our compiler always accepts the -pthread
argument, so THREADS_HAVE_PTHREAD_ARG can safely be set to ON.

Fixes:
 - http://autobuild.buildroot.org/results/f9e24d38d3aa4519615c5a0ae7f62af610df976f

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Arnout: set THREADS_HAVE_PTHREAD_ARG instead of THREADS_PTHREAD_ARG,
and set it to ON instead of OFF because we always have threads.]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>