Set fallback first_weekday_to 0 (fix oetiker#1012)

- Set first_weekday_to 0 (Sunday), when HAVE__NL_TIME_WEEK_1STDAY
  is not defined
- Fixes: oetiker#1012

src/rrd_graph.c

@@ -1568,7 +1568,7 @@ static int find_first_weekday(
         }
         first_weekday = (week_1stday + first_weekday - 1) % 7;
 #else
-        first_weekday = 1;
+        first_weekday = 0;
 #endif
     }
     return first_weekday;

src/rrd_rpncalc.c

@@ -564,7 +564,7 @@ static int find_first_weekday(void){
         }
         first_weekday=(week_1stday + first_weekday - 1) % 7;
 #else
-        first_weekday = 1;
+        first_weekday = 0;
 #endif
     }
     return first_weekday;