[sigc] exception_catch seems to be unusable

[Philip Langdale <plangdale vmware com>]

I was trying to use the exception_catch adaptor today and it seems to be
unusable.

The implementation of visit_each in exception_catch.h access the
catcher_ member of the functor but this is marked protected so
it's not going to work and the gcc errors bear this out.

The fixes are either to make catcher_ a public member or to
make visit_each a friend. I don't see friends used anywhere
else in the code and there's actually a comment in one place
saying that something is public to avoid writing a friend
declaration for a template (as would be the case here too).

So, it seems that catcher_ should be made public.

--phil