meson: fix build without cpp toolchain

We don't need a cpp toolchain for building glib so lets just
automatically disable tests requiring one when not available.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>

(cherry-picked from commit d0b9ebbaac)

gobject/tests/meson.build

@@ -29,9 +29,6 @@ marshalers_c = custom_target('marshalers_c',
 gobject_tests = {
   'qdata' : {},
   'boxed' : {},
-  'cxx' : {
-    'source' : ['cxx.cpp'],
-  },
   'enums' : {},
   'param' : {},
   'threadtests' : {},
@@ -59,6 +56,14 @@ gobject_tests = {
   'type-flags' : {},
 }
 
+if have_cxx
+  gobject_tests += {
+    'cxx' : {
+      'source' : ['cxx.cpp'],
+    },
+  }
+endif
+
 if cc.get_id() != 'msvc'
   gobject_tests += {'autoptr' : {}}
 endif