Don't depend on gtk+-3.0 if xmllint is found

When validating the .ui files, we need to determine the path to GTK 4's
relaxng schema. Previously this was erroneously looked up in the prefix
of GTK 3, which malcontent otherwise doesn't depend on.

Define the GTK dependency once, and use that variable everywhere.

libmalcontent-ui/meson.build

@@ -28,7 +28,7 @@ libmalcontent_ui_public_deps = [
   dependency('gio-2.0', version: '>= 2.44'),
   dependency('glib-2.0', version: '>= 2.54.2'),
   dependency('gobject-2.0', version: '>= 2.54'),
-  dependency('gtk4', version: '>= 4.6'),
+  gtk_dep,
   libadwaita_dep,
   libmalcontent_dep,
 ]
@@ -82,7 +82,7 @@ libmalcontent_ui_gir = gnome.generate_gir(libmalcontent_ui,
 
 xmllint = find_program('xmllint', required: false)
 if xmllint.found()
-  gtk_prefix = dependency('gtk+-3.0').get_variable(pkgconfig: 'prefix')
+  gtk_prefix = gtk_dep.get_variable(pkgconfig: 'prefix')
   test(
     'validate-ui', xmllint,
     args: [

malcontent-control/meson.build

@@ -27,7 +27,7 @@ malcontent_control = executable('malcontent-control',
     dependency('gio-2.0', version: '>= 2.44'),
     dependency('glib-2.0', version: '>= 2.54.2'),
     dependency('gobject-2.0', version: '>= 2.54'),
-    dependency('gtk4', version: '>= 4.6'),
+    gtk_dep,
     dependency('polkit-gobject-1'),
     libadwaita_dep,
     libmalcontent_dep,
@@ -79,7 +79,7 @@ endif
 
 xmllint = find_program('xmllint', required: false)
 if xmllint.found()
-  gtk_prefix = dependency('gtk+-3.0').get_variable(pkgconfig: 'prefix')
+  gtk_prefix = gtk_dep.get_variable(pkgconfig: 'prefix')
   test(
     'validate-ui', xmllint,
     args: [

meson.build

@@ -134,6 +134,7 @@ else
                        'Malcontent-' + libmalcontent_api_version + '.typelib']
 endif
 if get_option('ui').enabled()
+  gtk_dep = dependency('gtk4', version: '>= 4.6')
   libadwaita_dep = dependency(
     'libadwaita-1',
     version: '>= 1.1',