time-util: assume CLOCK_BOOTTIME always exists

Let's raise our supported baseline a bit: CLOCK_BOOTTIME started to work
with timerfd in kernel 3.15 (i.e. back in 2014), let's require support
for it now.

This will raise our baseline only modestly from 3.13 → 3.15.

src/core/timer.h

@@ -64,7 +64,7 @@ struct Timer {
         char *stamp_path;
 };
 
-#define TIMER_MONOTONIC_CLOCK(t) ((t)->wake_system && clock_boottime_supported() ? CLOCK_BOOTTIME_ALARM : CLOCK_MONOTONIC)
+#define TIMER_MONOTONIC_CLOCK(t) ((t)->wake_system ? CLOCK_BOOTTIME_ALARM : CLOCK_MONOTONIC)
 
 void timer_free_values(Timer *t);