meson: avoid ternary operator bug
Old meson versions (I don't remember which ones, but at least 0.49.2 which we use on the 20.1 branch is affected) have a bug where the ternary operator sometimes makes meson crash.
Let's replace it with a plain
if to fix this.
Fixes: 55765f80 ("util/u_thread: include pthread_np.h if found")