cmake: detect librt for POSIX time functions

cmake: detect librt for POSIX time functions

per clock_gettime(3),

> On POSIX systems on which these functions are available, the symbol
> _POSIX_TIMERS is defined in <unistd.h> to a value greater than 0.

and

> Most systems require the program be linked with the librt library to
> use these functions.

so, we should detect this symbol and link against librt for using these
functions.

in this change, librt is linked for checking the existence of
clock_gettime(), if it exists. RT_LIBRARY is defined.

Signed-off-by: Kefu Chai <kchai@redhat.com>