BUILD: Fix linking with librt
    
    The posix realime extensions defines timer_* functions
    but it does not mention library with these functions.
    http://www.unix.org/version2/whatsnew/realtime.html
    
    The autoconf macro AC_SEARCH_LIBS firstly check the function
    timer_create with no libraries, then for each library listed
    in 2nd parameter. Possible libraries librt and libposix4
    were used in nspr for similar detection.
    
    Reviewed-by: Joakim Tjernlund <joakim.tjernlund@infinera.com>
    (cherry picked from commit 6d11fdcd8ef05000dd20b3431f8491790f99a802)