Removed threading package option.

It didn't do what I thought it did.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 96f322f..08b319b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -30,7 +30,6 @@
     find_package(OpenGL REQUIRED)
 endif()
 
-set(CMAKE_THREAD_PREFER_PTHREAD OFF)
 find_package(Threads REQUIRED)
 
 #--------------------------------------------------------------------