pthread_cond_timedwait_relative() implemented

git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@6534 4407c894-4637-0410-b4f5-ada5f102cad1
Adjust header to fit more msdn-documentation.


git-svn-id: https://svn.code.sf.net/p/mingw-w64/code@6535 4407c894-4637-0410-b4f5-ada5f102cad1