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

[Jim Meyering]

Hello,
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:
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