from gdb 7.3.1 source code when opened as standalone files:

mi-getopt.h contains declaration:
extern int mi_getopt (const char *prefix, int argc, char **argv,
		      struct mi_opt *opt, int *optind, char **optarg);

mi-getopt.c contains definition:
int
mi_getopt (const char *prefix,
	   int argc, char **argv,
	   struct mi_opt *opts,
	   int *optind, char **optarg)
{
  char *arg;
  struct mi_opt *opt;
...

Ctrl+click on the definition goes to the header file but Ctrl+click on the declaration in .h file goes nowhere.
In classes view I can see that definition belongs to mi-getopt.c but declaration belongs to another .c file.