Determine at runtime which clockid_t to use for clock_gettime

Determine at runtime which clockid_t to use for clock_gettime

The cygwin platform (perhaps others) does not support
CLOCK_MONOTONIC in clock_gettime. However, it does support
CLOCK_REALTIME. Instead of having every call to clock_gettime
check which clock to use, check once and cache the result.

The only two clock options which seemed useful are CLOCK_MONOTONIC
and CLOCK_REALTIME. If others are available in the future
(or another clock type seems a good alternative if these two are
missing on a system) it can be added later.

git-svn-id: svn+ssh://svn.code.sf.net/p/check/code/trunk@787 64e312b2-a51f-0410-8e61-82d0ca0eb02a