Unit Tests: get ct compiling on CentOS-5

Fix some bugs that were preventing compilation (at least on CentOD 5).

Add `-lrt` to load the rt libary to prevent this error:
> ct.c:(.text+0x12): undefined reference to `clock_gettime'

Include `time.h` to prevent this error:
> ct/ct.c:47: error: ‘CLOCK_MONOTONIC’ undeclared (first use in this function)

Perform a cast to prevent this error: (on a 64bit arch)
> ct/ct.c:420: warning: format ‘%10lld’ expects type ‘long long int’, but argument 3 has type ‘int64

Don't use an expression in a const definition to prevent this error:
> ct/ct.c:28: error: initializer element is not constant

See pull kr/ct#7