-Wdeprecated-copy does find some real bugs, but it also complains
about a lot of reasonable code for which the implicitly declared copy
ctor/op= are fine oven though the class has a user-defined destructor:
this situation is only problematic if the destructor releases
resources held in one of the non-static data members.

So, this patch reins it in somewhat: first by moving from -Wall to
-Wextra, and then also only complaining if the other copy op is
user-declared.  The old behavior can be explicitly requested with
-Wdeprecated-copy-dtor.

Tested x86_64-pc-linux-gnu, applying to trunk.