posix.time: predicate clock_getres et.al. on _POSIX_TIMERS.

Close #295.
musl libc implements clock_getres and clock_gettime, and defines
_POSIX_TIMERS, but does not declare _XOPEN_REALTIME because it
does not implement the whole realtime API.
* lukefile (modules['posix.time'].libraries): Use _POSIX_TIMERS,
which is a subset of _XOPEN_REALTIME sufficient for clock_getres
and clock_gettime in the host libc.
* ext/posix/time.c (CLOCK_MONOTONIC, CLOCK_PROCESS_CPUTIME_ID)
(CLOCK_REALTIME, Pclock_getres, Pclock_gettime): Change preprocessor
guards from _XOPEN_REALTIME to _POSIX_TIMERS.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>

ext/posix/time.c

@@ -115,7 +115,7 @@ pushtm(lua_State *L, struct tm *t)
 }
 
 
-#if defined _XOPEN_REALTIME && _XOPEN_REALTIME != -1
+#if defined _POSIX_TIMERS
 /***
 Find the precision of a clock.
 @function clock_getres
@@ -326,7 +326,7 @@ Ptime(lua_State *L)
 
 static const luaL_Reg posix_time_fns[] =
 {
-#if defined _XOPEN_REALTIME && _XOPEN_REALTIME != -1
+#if defined _POSIX_TIMERS
 	LPOSIX_FUNC( Pclock_getres	),
 	LPOSIX_FUNC( Pclock_gettime	),
 #endif
@@ -348,7 +348,7 @@ luaopen_posix_time(lua_State *L)
 	lua_pushstring(L, LPOSIX_VERSION_STRING("time"));
 	lua_setfield(L, -2, "version");
 
-#if defined _XOPEN_REALTIME && _XOPEN_REALTIME != -1
+#if defined _POSIX_TIMERS
 	LPOSIX_CONST( CLOCK_MONOTONIC		);
 	LPOSIX_CONST( CLOCK_PROCESS_CPUTIME_ID	);
 	LPOSIX_CONST( CLOCK_REALTIME		);

lukefile

@@ -100,7 +100,7 @@ modules  = {
    ['posix.time']          = {
       libraries = {
          {
-            ifdef          = '_XOPEN_REALTIME',
+            ifdef          = '_POSIX_TIMERS',
             include        = 'unistd.h',
             checksymbol    = 'clock_gettime',
             library        = 'rt',