clock_gettime replacement (2.1.0 Release won't build on Mac)

Player 2.1.0 Release does not build on Mac OS X. This is due to new timeout code in messages.cc that uses the Linux/POSIX function clock_gettime. Mac does not support clock_gettime.

I've taken a replacement code for clock_gettime from a patch to the Orocos Project and put it into Player's replace system. I've emailed the patch author to confirm that the function is under the GPL, but I think it is safe to assume it is as Orocos is under the GPL and the function is rather short/trivial.

It'd be nice if we could get a 2.1.1 released soon so that Mac users aren't stuck with either rc2 or patching 2.1.0.

Apply with:
patch -p1 < mac_clock_gettime.diff

Then regeneration the configure script:
autoreconf -i