Prepend isnan with :: to satisfy CentOS compiler.

src/units.cc

@@ -475,7 +475,7 @@ string strWithUnitLowerCase(double v, Unit u)
 
 string valueToString(double v, Unit u)
 {
-    if (isnan(v))
+    if (::isnan(v))
     {
         return "null";
     }

src/util.cc

@@ -1365,7 +1365,7 @@ string strdate(struct tm *date)
 
 string strdate(double v)
 {
-    if (isnan(v)) return "null";
+    if (::isnan(v)) return "null";
     struct tm date;
     time_t t = v;
     localtime_r(&t, &date);
@@ -1381,7 +1381,7 @@ string strdatetime(struct tm *datetime)
 
 string strdatetime(double v)
 {
-    if (isnan(v)) return "null";
+    if (::isnan(v)) return "null";
     struct tm datetime;
     time_t t = v;
     localtime_r(&t, &datetime);