[libpeas] meson: Fix disabling gtk_doc

[Christian Hergert <chergert src gnome org>]

commit 1be9b5a51ca4200d9cff69642368c24225b58dd5
Author: Matt Turner <mattst88 gmail com>
Date:   Mon Mar 21 19:13:04 2022 -0700

    meson: Fix disabling gtk_doc
    
    Without this, even passing -Dgtk_doc=false will cause documentation to
    be built if gi-docgen is available.
    
    Fixes: 82119a6 ("gi-docgen: Initial commit")

 meson.build | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/meson.build b/meson.build
index c77b3524..91b8ca8f 100644
--- a/meson.build
+++ b/meson.build
@@ -251,7 +251,7 @@ configure_file(
 )
 
 # Options
-build_gtk_doc = gi_docgen_dep.found()
+build_gtk_doc = get_option('gtk_doc') and gi_docgen_dep.found()
 
 install_glade_catalog = get_option('glade_catalog')
 if install_glade_catalog and not gladeui_dep.found()