Fix relocation of directories in the MinGW build.

windows-nat.c (windows_get_absolute_argv0): New function.
 windows-nat.h: Add its prototype.
 main.c (get_init_files): Use filename_ncmp instead of strncmp.
 Use IS_DIR_SEPARATOR instead of looking for a character inside
 SLASH_STRING.  Include filenames.h.
 (captured_main) [__MINGW32__]: Make argv[0] absolute, so that
 relocate_gdb_directory works when passed gdb_program_name.
 Include windows-nat.h.

gdb/windows-nat.c

@@ -597,6 +597,18 @@ failed:
   return 0;		/* failure */
 }
 
+/* Return an absolute file name of the running GDB, if possible, or
+   ARGV0 if not.  The return value is in malloc'ed storage.  */
+char *
+windows_get_absolute_argv0 (const char *argv0)
+{
+  char full_name[PATH_MAX];
+
+  if (GetModuleFileName (NULL, full_name, PATH_MAX))
+    return xstrdup (full_name);
+  return xstrdup (argv0);
+}
+
 /* Encapsulate the information required in a call to
    symbol_file_add_args.  */
 struct safe_symbol_file_add_args