Explicitly check for pthread library, and link against it.

CMakeLists.txt

@@ -170,6 +170,11 @@ IF(UNIX)
         IF(HAVE_PTHREAD_H)
             SET(PHYSFS_HAVE_THREAD_SUPPORT TRUE)
         ENDIF(HAVE_PTHREAD_H)
+
+        FIND_LIBRARY(PTHREAD_LIBRARY pthread)
+        IF(PTHREAD_LIBRARY)
+            SET(OPTIONAL_LIBRARY_LIBS ${OPTIONAL_LIBRARY_LIBS} ${PTHREAD_LIBRARY})
+        ENDIF(PTHREAD_LIBRARY)
     ENDIF(BEOS)
 ENDIF(UNIX)