add a gvLayoutDone function to the gvc lib

add a gvLayoutDone function to the gvc lib

An upcoming commit in the series need to perform this check from
C++. Using the LAYOUT_DONE macro directly would give the warning
below. It would be possible to change the underlying C functions to
take a const char * instead, but exposing this new function seems
better than using a macro in C++ anyway.

The warning that would have been the result otherwise:

In file included from /home/magjac/graphviz/lib/gvc++/GVContext.h:5,
                 from /home/magjac/graphviz/lib/gvc++/GVLayout.cpp:6:
/home/magjac/graphviz/lib/gvc++/GVLayout.cpp: In constructor ‘GVC::GVLayout::GVLayout(const std::shared_ptr<GVC::GVContext>&, const std::shared_ptr<CGraph::AGraph>&, const string&)’:
/home/magjac/graphviz/lib/gvc/gvc.h:38:38: warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
   38 | #define LAYOUT_DONE(g) (agbindrec(g, "Agraphinfo_t", 0, TRUE) && GD_drawing(g))
      |                                      ^~~~~~~~~~~~~~
/home/magjac/graphviz/lib/gvc++/GVLayout.cpp:18:9: note: in expansion of macro ‘LAYOUT_DONE’
   18 |     if (LAYOUT_DONE(g->c_struct())) {}
      |         ^~~~~~~~~~~