Nested C class setters restored in c++out mode for Octave

Suitable casts are required so that assignment of instances of nested classes
work as the nested class is duplicated in the global namespace, eg:

struct Outer {
  struct Nested {
    int bar;
  } bar_instance;
};

Outer.bar_instance can now be assigned to.
Add missing header to new source file
Ensure -c++out is not used with -c++

Error checking for this combination implemented as well as correcting
Octave

Also refactor, replacing CPlusPlusOut variable with cparse_cplusplusout
for an implementation which more closely resembles cparse_cplusplus which
is also required in both .c and .cxx files.