Add a missing -pthread to fix compile with slibtool.
When compiling wayland with slibtool instead of GNU libtool it will fail building libtest_runner with an undefined reference to pthread_join@@GLIBC_2.2.5. This is because -pthread (Or -lpthread) is missing from display_test. If its added the build succeeds as expected with slibtool and continues to work with libtool. Its likely that libtool is hiding this failure by silently adding the missing flag which is not uncommon... Exposed in commit aa51a833. Fixes #91 Signed-off-by: orbea <firstname.lastname@example.org>