Fix configure and build errors on OS X

[clemensg]

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