autoconf check for nanosleep()

[Drazen Kacar <dave srce hr>]

I found autoconf check for nanosleep() in esound, but it's probably
present in other modules. Checking if that function is in libposix4 is
most likely Solaris specific, at least as far as I know. The slight
problem with this is that Solaris API moved that function to librt, so
that one should be checked first and libposix4 should be checked if that
fails. libposix4 still contains the function, but it will be dropped
in a future release, most likely.

My knowledge of autoconf is next to nothing, so if there is a more appropriate
forum for this report, please forward the mail there.

-- 
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