Fix libgltf dbgutil build

The simpler version of the master fix
  commit 1f23cc159de5e8584b9354a42d431356530d1ce0

This just changes the environment visibility of CPPFLAGS by
exporting it, so $(MAKE) and configure actually see it.

Change-Id: I6d508cbfbee26cdb3062f0628ee8151be4f11881
Reviewed-on: https://gerrit.libreoffice.org/10788
Reviewed-by: Zolnai Tamás <tamas.zolnai@collabora.com>
Tested-by: Zolnai Tamás <tamas.zolnai@collabora.com>