[gimp] extensions: do not build Vala plug-in without GObject Introspection.

[Jehan <jehanp src gnome org>]

commit 326b2feddb4fd7a0135b93c48a7728f0e0e2c2fb
Author: Jehan <jehan girinstud io>
Date:   Sun Oct 18 00:34:02 2020 +0200

    extensions: do not build Vala plug-in without GObject Introspection.
    
    Though GObject Introspection is normally not an option, the only case we
    don't build it yet is when cross-compiling (as we haven't found the
    right way to do it). So let's not build the Goat Exercise plug-in in
    Vala in such case as we needed the introspected libgimp.

 extensions/goat-exercises/meson.build | 2 +-
 libgimp/meson.build                   | 2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/extensions/goat-exercises/meson.build b/extensions/goat-exercises/meson.build
index 5169d56930..decbcdd41d 100644
--- a/extensions/goat-exercises/meson.build
+++ b/extensions/goat-exercises/meson.build
@@ -42,7 +42,7 @@ install_data(
 
 # Vala version
 
-if have_vala
+if have_vala and have_gobject_introspection
   exe = executable('goat-exercise-vala',
     'goat-exercise-vala.vala',
     include_directories: [ rootInclude, ],
diff --git a/libgimp/meson.build b/libgimp/meson.build
index a346e6f965..2126649c9b 100644
--- a/libgimp/meson.build
+++ b/libgimp/meson.build
@@ -383,6 +383,7 @@ libgimpui_introspectable_files = [
 ]
 
 if meson.is_cross_build()
+  have_gobject_introspection = false
   gobject_introspection_warning = '\n      '.join(['',
     'GObject Introspection is disabled while cross-compiling because',
     'GI tools do not manage to properly generate introspection data',
@@ -393,6 +394,7 @@ if meson.is_cross_build()
   warning(gobject_introspection_warning)
   warnings += gobject_introspection_warning
 else
+  have_gobject_introspection = true
   libgimp_gir_includes = [
     'Babl-0.1',
     'cairo-1.0',