Cycles: Solve linking error caused by missing pthreads library

Not sure why it worked on Debian but didn't work on Arch, could have
been some indirect link dependency or so.

Anyway, we explicitly depends on pthreads, so need to do corresponding
find_package().

intern/cycles/cmake/external_libs.cmake

@@ -46,6 +46,10 @@ if(CYCLES_STANDALONE_REPOSITORY)
 		include(precompiled_libs)
 	endif()
 
+	# PThreads
+	find_package(Threads REQUIRED)
+	set(PTHREADS_LIBRARIES ${CMAKE_THREAD_LIBS_INIT})
+
 	####
 	# OpenGL