Submitted upstream at:
https://bitbucket.org/multicoreware/x265/pull-requests/31

diff -u -r ../x265_2.2/source/CMakeLists.txt ./source/CMakeLists.txt
--- ../x265_2.2/source/CMakeLists.txt	2016-12-23 00:57:39.000000000 -0500
+++ ./source/CMakeLists.txt	2017-01-03 18:42:19.823674722 -0500
@@ -80,7 +80,10 @@
 endif()
 
 if(UNIX)
-    list(APPEND PLATFORM_LIBS pthread)
+    find_library(LIBPTHREAD pthread)
+    if(LIBPTHREAD)
+         list(APPEND PLATFORM_LIBS pthread)
+    endif()
     find_library(LIBRT rt)
     if(LIBRT)
         list(APPEND PLATFORM_LIBS rt)