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

[clemensg]

Any thoughts?

[PiotrSikora]

I'm wondering why preadv and pwritev are used in the first place? It looks that the code would be much simpler and more portable with pread and pwrite.

[clemensg]

Yes, I think so too. I changed the PR!

[ioerror]

How does this impact other OS X builds? Could you please report on any other impacted platforms as well?

[bfontaine]

On OS X 10.11.2 (15C50) tlsdate 0.0.13 doesn’t build without this patch.

[vapier]

your change is not equivalent -- it's changing the file offset when you use lseek. please change the code to use pread/pwrite instead.

you should also split the commits up -- one to change the configure script and one to update the pread/pwrite logic

[clemensg]

Hi, sorry for the delay, I totally forgot about this PR.

@vapier , you are right. I modified the patch to use pread and pwrite and split up the commit.

@ioerror , I can only test ArchLinux, OS X 10.10 and OS X 10.11 but nothing older than that.
Maybe someone else has an older OS X machine to test it with?

[ilovezfs]

@ioerror @clemensg ping on this PR