Fix configure and build errors on OS X Yosemite

Hi,

On OS X 10.10 I can only find `readv` and `writev` but not `preadv` or `pwritev`, so I substituted `preadv` with `lseek`+`readv` if `preadv` is not available, and did the same for `pwritev`.
I also removed a seemingly redundant autoconf check for `clock_gettime`, which broke the OS X configuration. The same check already exists in the case statements for platforms, which need `clock_gettime`.

I tested my changes on OS X 10.10 (Yosemite) and Debian Wheezy (`make check` tests passed).
I hope I did not break the build on any other platform.

If there is a better way to fix the build errors on OS X, please comment and I will update the PR.
Thanks.

Regards,
Clemens