ortho: MSBuild: define GVDLL

ortho: MSBuild: define GVDLL

Without this change, an upcoming commit in this series that will use
GVDLL instead of _WIN32 to enable Microsoft-specific storage-class
attributes would cause this error:

neatogen.lib(neatosplines.obj) : error LNK2019: unresolved external symbol _orthoEdges referenced in function __spline_edges [C:\GitLab-Runner\builds\magjac\graphviz\plugin\neato_layout\gvplugin_neato_layout.vcxproj]

Since https://gitlab.com/graphviz/graphviz/-/merge_requests/2182,
setting GVDLL is the preferred single mechanism to enable
Microsoft-specific storage-class attributes.

Also towards
https://gitlab.com/graphviz/graphviz/-/issues/2058#note_678679545