Declare internal routine as static.

* This allows inlining, which the compiler must not use for
  extern functions.

src/mtime_julianDay.c

@@ -309,7 +309,7 @@ compareJulianDay(struct _julianday* jd1, struct _julianday* jd2)
 /*Internal function.*/
 /* Return jd1 >= jd2 */
 
-bool
+static bool
 internal_compareJulianDay(struct _julianday* jd1, struct _julianday* jd2)
 {
   if (jd1->day > jd2->day)