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>

gmodule/tests/meson.build

@@ -1,7 +1,4 @@
 gmodule_tests = {
-  'cxx' : {
-    'source' : ['cxx.cpp'],
-  },
   'module-test-library' : {
     'export_dynamic' : true,
     'source': 'module-test.c',
@@ -14,6 +11,14 @@ gmodule_tests = {
   },
 }
 
+if have_cxx
+  gmodule_tests += {
+    'cxx' : {
+      'source' : ['cxx.cpp'],
+    }
+  }
+endif
+
 module_suffix = []
 # Keep the autotools convention for shared module suffix because GModule
 # depends on it: https://gitlab.gnome.org/GNOME/glib/issues/520