Build failures on Mac OS X

[Alex Ray]

On Mac OS X, there are a couple of build failures caused by the macro `g_return_if_fail` instead of `g_return_val_if_fail` in contexts that return non void types.

gschem_options_dialog.c:219:3: error: non-void function 'create_grid_mode_widget' should return a value [-Wreturn-type]
  g_return_if_fail (dialog != NULL);
  ^
/usr/local/Cellar/glib/2.42.1/include/glib-2.0/glib/gmessages.h:373:3: note: expanded from macro 'g_return_if_fail'
         return; \
         ^
gschem_options_dialog.c:342:3: error: non-void function 'create_snap_mode_widget' should return a value [-Wreturn-type]
  g_return_if_fail (dialog != NULL);
  ^
/usr/local/Cellar/glib/2.42.1/include/glib-2.0/glib/gmessages.h:373:3: note: expanded from macro 'g_return_if_fail'
         return; \
         ^
19 warnings and 2 errors generated.

[Alex Ray (a-h2)]

Patched the offending locations, attached patch.

[Edward Hennessy (ehennes)]

Alex, Thank you for the patch. -Ed