[build] Copy cogl-enum-types.h under the guard
[build] Copy cogl-enum-types.h under the guard

We avoid rebuilding cogl-enum-types.h and cogl-enum-types.c by
using a "guard" -- a stamp file that will block Makefile. Since
we need cogl-enum-types.h into /clutter/cogl as well for the
cogl.h include to work, if we copy the cogl-enum-types.h
unconditionally it will cause a rebuild of the whole COGL; which
will cause a full rebuild.

To solve this, we can copy the header file when generating it
under the stamp file.

spatulasnout/cogl,gcampax/cogl,Distrotech/cogl,gcampax/cogl,spatulasnout/cogl,collects/cogl,djdeath/cogl-android,djdeath/cogl-android,Distrotech/cogl,collects/cogl,djdeath/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl,djdeath/cogl-android,djdeath/cogl,Distrotech/cogl,Distrotech/cogl