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 https://gitlab.freedesktop.org/wayland/wayland/issues/91



Signed-off-by: orbea <orbea@riseup.net>