Disable GCC's -Woverloaded-virtual in the configure+make build. Clang's is better.

Turns out Clang's -Woverloaded-virtual is enabled by -Wall in both CMake
and Configure builds. We were only explicitly specifying it (thus
enabling GCC's version of the warning) in the Configure build.

The specific case of interest is:

  struct base {
    virtual void func();
    virtual void func(int);
  };
  struct derived: base {
    virtual void func(); // GCC warns here, because this causes
                         // func(int) to be hidden
  };

I don't think that's worth getting fussed about (& Clang (indirectly
me... since I improved this warning in Clang) agrees or we would've made
the warning catch these cases.

Technically this could still lead to bugs/confusion if base had
func(int) and func(bool), derived overrode func(bool) and then a caller
with a derived object tried to call func(42) - it would silently call
func(bool). We should probably improve clang's warnings to catch this at
the call site at some point.