file_is_newer: fix compilation on Mac

OSX suddenly started failing the build due to implicit declaration of
file_is_newer. It was defined in ImageJ.c but only used in file-funcs.c

Fixed by moving to file-funcs and externalizing

Author: hinerm

src/main/c/ImageJ.c

@@ -595,15 +595,6 @@ static void show_commandline(struct options *options)
 	fputc('\n', stdout);
 }
 
-int file_is_newer(const char *path, const char *than)
-{
-	struct stat st1, st2;
-
-	if (stat(path, &st1))
-		return 0;
-	return stat(than, &st2) || st1.st_mtime > st2.st_mtime;
-}
-
 int handle_one_option(int *i, const char **argv, const char *option, struct string *arg)
 {
 	int len;

src/main/c/file-funcs.c

@@ -36,6 +36,16 @@
 
 #include <sys/stat.h>
 
+int file_is_newer(const char *path, const char *than)
+{
+	struct stat st1, st2;
+
+	if (stat(path, &st1))
+		return 0;
+	return stat(than, &st2) || st1.st_mtime > st2.st_mtime;
+}
+
+
 int find_file(struct string *search_root, int max_depth, const char *file, struct string *result)
 {
 	int len = search_root->length;

src/main/include/file-funcs.h

@@ -31,6 +31,7 @@
 
 #include "string-funcs.h"
 
+extern int file_is_newer(const char *path, const char *than);
 extern int find_file(struct string *search_root, int max_depth, const char *file, struct string *result);
 extern void detect_library_path(struct string *library_path, struct string *directory);