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>

gobject/tests/meson.build

@@ -38,9 +38,6 @@ gobject_tests = {
   },
   'basics-gobject' : {},
   'boxed' : {},
-  'cxx' : {
-    'source' : ['cxx.cpp'],
-  },
   'defaultiface' : {
     'source' : ['defaultiface.c', 'testmodule.c'],
   },
@@ -101,6 +98,14 @@ gobject_tests = {
   },
 }
 
+if have_cxx
+  gobject_tests += {
+    'cxx' : {
+      'source' : ['cxx.cpp'],
+    },
+  }
+endif
+
 if cc.get_id() != 'msvc'
   gobject_tests += {'autoptr' : {}}
 endif