clear USE_GTK_3 macro

[yetist]

Please review and test.

[alerque]

What is the advantage of checking for this repeatedly at compile time instead of once at configure time?

[yetist]

There is no need to define USE_GTK3 in the build system.

1. One reason is that the build may be failed.
   For example, when gtk-3.8 is installed in the system, the build system detects gtk+-3.0.pc, and then defines USE_GTK_3.
   But in the code, if USE_GTK3, we call the newly api that added after gtk-3.14.

2. Another reason is that we can clean up the code.

For example, after gtk-2.18, we can use gtk_widget_set_can_default, so we can delete other lines:

-#if GTK_CHECK_VERSION(2, 18, 0)
 	gtk_widget_set_can_default(button, TRUE);
-#elif defined(USE_GTK_3)
-	gtk_widget_set_can_default(button, 1);
-#else
-	GTK_WIDGET_SET_FLAGS(button, GTK_CAN_DEFAULT);
-#endif

3. Enhance strong maintainability

If you use USE_GTK_3, can you quickly know that the following changes are safe?

-#ifdef USE_GTK_3
 			gtk_combo_box_text_append_text((GtkComboBoxText *)ss.entryUpper, book);
 			gtk_combo_box_text_append_text((GtkComboBoxText *)ss.entryLower, book);
-#else
-			gtk_combo_box_append_text(GTK_COMBO_BOX(ss.entryUpper), book);
-			gtk_combo_box_append_text(GTK_COMBO_BOX(ss.entryLower), book);
-#endif