Upgrade #94: [Makefile] GIMP3 compatibility

 - Specify the batch script interpreter to use; GIMP3 provides
   multiple.

 - Use `gimp-console` instead of `gimp` to avoid a non-interactive
   mode issue[1].

This doesn't solve all compatibility problems with GIMP3 yet; we
also need to resolve the removal of a `gimp-image-get-active-layer`
API function in GIMP3.

The changes* here are intended to be backwards-compatible to GIMP2
and have been tested GIMP2.10.34 on Debian stable (bookworm), and
also with GIMP3.0.0 RC2 to build the OpenGFX 7.1 sources.

- [1] https://gitlab.gnome.org/GNOME/gimp/-/issues/12042

*NB: the `which` command has been superseded by `command -v` since
 v7.1 in VCS, so a small patch adjustment is required if backporting.

Makefile

@@ -97,8 +97,8 @@ GREP           ?= grep
 PYTHON         ?= python
 
 # Graphics processing
-GIMP           ?= $(shell command -v gimp)
-GIMP_FLAGS     ?= -n -i
+GIMP           ?= $(shell command -v gimp-console)
+GIMP_FLAGS     ?= -n -i --batch-interpreter=plug-in-script-fu-eval
 
 # NML
 NML            ?= nmlc