This patch enables an EGL extension, EGL_KHR_reusable_sync.
This new extension basically provides a way for multiple APIs or
threads to be excuted synchronously via a "reusable sync"
primitive shared by those threads/API calls.

This was implemented based on the specification at

https://www.khronos.org/registry/egl/extensions/KHR/EGL_KHR_reusable_sync.txt



v2
- use thread functions defined in C11/threads.h instead of
  using direct pthread calls
- make the timeout set with reference to CLOCK_MONOTONIC
- cleaned up the way expiration time is calculated
- (bug fix) in dri2_client_wait_sync, case EGL_SYNC_CL_EVENT_KHR
  has been added.
- (bug fix) in dri2_destroy_sync, return from cond_broadcast
  call is now stored in 'err' intead of 'ret' to prevent 'ret'
  from being reset to 'EGL_FALSE' even in successful case
- corrected minor syntax problems

v3
- dri2_egl_unref_sync now became 'void' type. No more error check
  is needed for this function call as a result.
- (bug fix) resolved issue with duplicated unlocking of display in
  eglClientWaitSync when type of sync is "EGL_KHR_REUSABLE_SYNC"

Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>