fixed compilation issue under linux

src/low_level_utils.cpp

@@ -9,6 +9,9 @@
 #include <iostream>
 #include <sys/stat.h>
 #include <sys/sysctl.h>
+#if !defined(_SC_PHYS_PAGES) and !__APPLE__
+    #include <sys/sysinfo.h>
+#endif
 #include "low_level_utils.h"
 
 using namespace std;
@@ -51,8 +54,7 @@ unsigned long getNodePhysMemory() {
 	mem=0;
     }
     mem /=  1 * 1024 * 1024 * 1024;
-#else
-#include <sys/sysinfo.h>
+#elif !defined(_SC_PHYS_PAGES) and !__APPLE__
     struct sysinfo si;
     int ret=sysinfo(&si);
     if (ret==-1) {