Consistently treat gdkenumtypes.[hc] as source files

Consistently treat gdkenumtypes.[hc] as source files

Always generate gdk/gdkenumtypes.c in the source tree, and fix path in the
introspection sources. With this, gdkenumtypes.c is correctly included with
separate build trees, too. This fixes missing GTypes in the .gir when using a
separate build tree.

Also remove the gdk/stamp-gdkenumtypes.h target. It was forcing the
regeneration of gdk/gdkenumtypes.h even if it was already present and current
in the source tree.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=647729
Bug-Ubuntu: https://launchpad.net/bugs/769256

Gbp-Pq: Name 018_gdkenumtypes.c_location.patch