systemtime: Fix includes and check for _POSIX_TIMERS

Fix missing include of windows.h on windows platform. Change check
for _POSIX_TIMERS to check that it is greater than 0. On OS X, the
POSIX timers are not available and _POSIX_TIMERS is -1.

src/systemtime.c

@@ -18,17 +18,22 @@
 */
 
 #include <stdio.h>
+#include <unistd.h>
 
 #ifndef _WIN32
-# ifdef _POSIX_TIMERS
+# if _POSIX_TIMERS > 0
 #  include <time.h>
 # else
 #  include <sys/time.h>
 # endif
+#else
+# include <windows.h>
 #endif
 
 #include "systemtime.h"
 
+
+/* Return current time in T as the number of seconds since the epoch. */
 int
 systemtime_get_time(double *t)
 {
@@ -41,7 +46,7 @@ systemtime_get_time(double *t)
 
 	/* FILETIME is tenths of microseconds since 1601-01-01 UTC */
 	*t = (i.QuadPart / 10000000.0) - 11644473600.0;
-#elif defined(_POSIX_TIMERS) /* POSIX timers */
+#elif _POSIX_TIMERS > 0 /* POSIX timers */
 	struct timespec now;
 	int r = clock_gettime(CLOCK_REALTIME, &now);
 	if (r < 0) {

src/systemtime.h

@@ -14,18 +14,12 @@
    You should have received a copy of the GNU General Public License
    along with Redshift.  If not, see <http://www.gnu.org/licenses/>.
 
-   Copyright (c) 2010  Jon Lund Steffensen <jonlst@gmail.com>
+   Copyright (c) 2010-2014  Jon Lund Steffensen <jonlst@gmail.com>
 */
 
 #ifndef REDSHIFT_SYSTEMTIME_H
 #define REDSHIFT_SYSTEMTIME_H
 
-#ifndef _WIN32
-# include <time.h>
-#else
-# include <windows.h>
-#endif
-
 
 int systemtime_get_time(double *now);