use a virtual do_gc() function instead of virtual destructor; gives more freedom in choice of memory allocation

add debug information (string type) for rcu data structures

clang complains that these are unnecessary for Linux and from reading
the gcc manual it looks like that might now be unnecessary for OS X.

"C++11 requires lambda with omitted result type to consist of a single
return statement"

The gcc manual states that specifying packed "for struct and union
types is equivalent to specifying the packed attribute on each of the
structure or union members."  lwIP always uses PACK_STRUCT_STRUCT when
it uses PACK_STRUCT_FIELD within the struct definition.

"C++11 requires lambda with omitted result type to consist of a single
return statement"

"C++11 requires lambda with omitted result type to consist of a single
return statement"

"cannot initialize a parameter of type 'struct range *' with an rvalue
of type 'int'"

"C++11 requires lambda with omitted result type to consist of a single
return statement"

Note on how to build g++ cross compiler on mac