When enabling loggint output, e.g.
GEGL_DEBUG=all GEGL_USE_OPENCL=yes ninja test

calls to GEGL_NOTE that use %s parameters produce garbage output
instead of the strings, e.g.
GEGL-Message: 14:38:47.453: [ opencl ] ../../gegl/gegl/gegl-debug.h:145:
Platform Name: \u0018\xc1\xe3C\xff\u007f

Since this is inside a function using varargs, change the call to
make that function aware of that, so use
g_strdup_vprintf instead of g_strdup_printf