build: error out on inappropriate use of dout()
Right now, if you use dout() instead of ldout() in a library context, the compilation phase goes fine and then you get a lovely linker error about not being able to find "g_ceph_context" that doesn't point at the real issue at all.
This is because dout() is implicitly referencing g_ceph_context, but we don't supply globals for our libraries. I think all we need to do to make this work out better is to #undef dout in all the library context headers. Then you'll get a compile error you can do something with instead of a linker error you need to just know about ahead of time. ;)