circogen: autotools: define NEATOGEN_EXPORTS

circogen: autotools: define NEATOGEN_EXPORTS

circogen is part of the neato_layout plugin.

Fixes these errors using autotools with MinGW:

C:/tools/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/10.3.0/../../../../x86_64-w64-mingw32/bin/ld.exe: ../../lib/circogen/.libs/libcircogen_C.a(circularinit.o): in function `circular_init_node_edge':
C:\Users\magja\graphviz\lib\circogen/circularinit.c:49: undefined reference to `__imp_neato_init_node'
C:/tools/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/10.3.0/../../../../x86_64-w64-mingw32/bin/ld.exe: ../../lib/circogen/.libs/libcircogen_C.a(circularinit.o): in function `circo_layout':
C:\Users\magja\graphviz\lib\circogen/circularinit.c:249: undefined reference to `__imp_spline_edges'

Towards https://gitlab.com/graphviz/graphviz/-/issues/2058