Define monotonic and realtime clock, even if librt is available

Define monotonic and realtime clock, even if librt is available

Cygwin has librt and implements clock_gettime(), but does not
define CLOCK_MONOTONIC.

Moving these definitions outside of the check for librt, as they
may be needed even if the system has librt.

git-svn-id: svn+ssh://svn.code.sf.net/p/check/code/trunk@818 64e312b2-a51f-0410-8e61-82d0ca0eb02a