mobileOrg auto-sync / gdb update

- Enabled auto sync of the content related to mobileOrg, as per
  https://github.com/matburt/mobileorg-android/wiki/FAQ#wiki-How_do_I_get_orgmode_to_execute_orgmobilepush_automatically
- After installing a newer version of gdb, which I only did locally so
  it won't mess up my OS X settings, I added a line of code to tell
  emacs which gdb tool to use.