ortho: use GVDLL instead of _WIN32 to enable storage-class attributes

ortho: use GVDLL instead of _WIN32 to enable storage-class attributes

The immediate reason for this change is to enable Windows-specific
storage-class attributes for autotools builds with Cygwin. The _WIN32
symbol is not defined when using Cygwin.

Since https://gitlab.com/graphviz/graphviz/-/merge_requests/2182,
setting GVDLL is the preferred single mechanism to enable those
attributes, rather than using _WIN32 and __CYGWIN__ plus some other
symbol to distinguish between static and dynamic-link/shared
libraries.

This is the first step of two in fixing these errors in autotools
builds with Cygwin:

/usr/lib/gcc/x86_64-pc-cygwin/10/../../../../x86_64-pc-cygwin/bin/ld: ../../lib/dotgen/.libs/libdotgen_C.a(dotsplines.o): in function `_dot_splines':
/home/magja/graphviz/lib/dotgen/dotsplines.c:334: undefined reference to `orthoEdges'
/usr/lib/gcc/x86_64-pc-cygwin/10/../../../../x86_64-pc-cygwin/bin/ld: /home/magja/graphviz/lib/dotgen/dotsplines.c:337: undefined reference to `orthoEdges'

The Windows-specific storage-class attributes are however still not
correct for Cygwin autotools builds, since GVC_EXPORT is currently not
set in the autotools builds. The error is now instead:

/usr/lib/gcc/x86_64-pc-cygwin/10/../../../../x86_64-pc-cygwin/bin/ld: ../../lib/dotgen/.libs/libdotgen_C.a(dotsplines.o): in function `_dot_splines':
/home/magja/graphviz/lib/dotgen/dotsplines.c:334: undefined reference to `__imp_orthoEdges'
/usr/lib/gcc/x86_64-pc-cygwin/10/../../../../x86_64-pc-cygwin/bin/ld: /home/magja/graphviz/lib/dotgen/dotsplines.c:337: undefined reference to `__imp_orthoEdges'

The next commit in this series will fix this.

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