[gimp] INSTALL: update GEGL build instructions.

[Jehan <jehanp src gnome org>]

commit 3f597691203c7c51f673c4b5a3a05d2ee44789b3
Author: Jehan <jehan girinstud io>
Date:   Tue Nov 17 03:55:42 2020 +0100

    INSTALL: update GEGL build instructions.
    
    Recently updated my distrib, so it's a good time to see what is wrong in
    our instructions. In particular, we must build GEGL with Cairo,
    otherwise gegl:npd is not built (and it's a mandatory operation for
    GIMP).

 INSTALL.in | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)
---
diff --git a/INSTALL.in b/INSTALL.in
index 830037d0b6..c89ae7b10b 100644
--- a/INSTALL.in
+++ b/INSTALL.in
@@ -45,9 +45,12 @@ header files installed.
        https://gitlab.gnome.org/GNOME/babl.git
        https://gitlab.gnome.org/GNOME/gegl.git
 
-     Note: install GEGL with libumfpack (SuiteSparse) for alternative Matting
-     engine "gegl:matting-levin" and OpenEXR library for OpenEXR format
-     support.
+     GEGL must be built with Cairo support, i.e. -Dcairo=enabled option (required
+     for some mandatory operations such as "gegl:npd").
+
+     Optional: build GEGL with libumfpack (SuiteSparse) (`-Dumfpack=enabled`)
+     for alternative Matting engine "gegl:matting-levin" and OpenEXR library
+     (`-Dopenexr=enabled`) for OpenEXR format support.
 
   4. You need to have installed GTK+ version @GTK_REQUIRED_VERSION@ or newer.
      GIMP also needs a recent version of GLib (>= @GLIB_REQUIRED_VERSION@), GDK-Pixbuf