Clocksource: use a better clock if available.

clock_gettime() is a far better clock than gettimeofday().

Even better than clock_gettime() is that you can select either
CLOCK_MONOTONIC, or even CLOCK_MONOTONIC_RAW. These clocks offer
high precision time. And the _RAW variant will never roll back
due to NTP drift or daylight savings, or otherwise.

I've adjusted this code to select the right clock method auto-
matically based on what's available in the OS. This means that
if you're running a very old linux version, MacOS or other,
you will automatically get the best clocksource available.

I've tested all Linux clocksources by selectively compiling and
running a 10k+ timer test suite. In all cases I confirmed that
the 3 POSIX Linux clocksources worked properly, and were
selected properly.

I've modified the OS X compile path to use the high-res clock
source for all time functions, but I can't confirm it works or
that it compiles.

As for WIN32, I confirmed that the used clocksource is indeed
a Monotonic clocksource, so good news: that code section appears
to be exactly what it should be.

doc/lua_api.txt

@@ -1727,7 +1727,7 @@ Helper functions
 * `minetest.is_yes(arg)`
     * returns whether `arg` can be interpreted as yes
 * `minetest.get_us_time()`
-    * returns time with microsecond precision
+    * returns time with microsecond precision. May not return wall time.
 * `table.copy(table)`: returns a table
     * returns a deep copy of `table`