INSTALL, libgimp: document a bit better the GIR cross-tool exception.

INSTALL.in

@@ -48,6 +48,14 @@ header files installed.
      GEGL must be built with Cairo support, i.e. -Dcairo=enabled option (required
      for some mandatory operations such as "gegl:npd").
 
+     Introspection must be enabled for both babl and GEGL with
+     -Denable-gir=true and -Dintrospection=true respectively. The only
+     case where we don't build GIR data is when cross-compiling on meson
+     because of the difficulty to make cross-tools for GObject
+     Introspection. Nevertheless if you have working GIR cross-tools,
+     you can force the expected behaviour with GIMP's meson option
+     -Dcan-crosscompile-gir=true
+
      Optional:
 
      - build GEGL with libumfpack (SuiteSparse) (`-Dumfpack=enabled`)

libgimp/meson.build

@@ -391,7 +391,9 @@ if meson.is_cross_build() and not get_option('can-crosscompile-gir')
     'GI tools do not manage to properly generate introspection data',
     'while cross-compiling. Nevertheless we consider our introspected',
     'API as a core component of GIMP, thus you are expected to build',
-    'this data natively.'
+    'this data natively.',
+    'To forcefully build with GObject Introspection, set:',
+    '-Dcan-crosscompile-gir=true'
   ])
   warning(gobject_introspection_warning)
   warnings += gobject_introspection_warning