[PATCH] configure.ac: improve check for clock_gettime library

[NIIBE Yutaka]

On 06/06/2015 12:49 AM, Jim Meyering wrote:
> I noticed the clock_gettime-vs-librt problem with npth-1.2 on a CentOS6
> system and fixed it, but then noticed that the first post-release patch also
> made a change on that front. I have rebased on top of that.
> 
> I have attached an additional patch:

Thank you.  Applied and pushed.
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