I just noticed that ipmidi is not linked against libws2_32.a and thus Csound6 can't be compiled with ipmidi enabled.
from InOut/CMakeLists.txt line 167:
if(USE_IPMIDI)
make_plugin(ipmidi ipmidi.c "${ipmidi_LIBS}")
endif()
I propose to change it to
if(USE_IPMIDI)
make_plugin(ipmidi ipmidi.c "${ipmidi_LIBS}")
target_link_libraries(ipmidi ws2_32)
endif()