Link pthread for product_threaded test

test/CMakeLists.txt

diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 317a6ba..0614a20 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -234,7 +234,7 @@
 ei_add_test(product_trsolve)
 ei_add_test(product_mmtr)
 ei_add_test(product_notemporary)
-ei_add_test(product_threaded)
+ei_add_test(product_threaded "-pthread" "${CMAKE_THREAD_LIBS_INIT}")
 ei_add_test(stable_norm)
 ei_add_test(permutationmatrices)
 ei_add_test(bandmatrix)