It is puzzling that Executable_soffice.bin.mk contains the line
$(eval $(call gb_Executable_set_targettype_gui,$(sofficebin),YES))
yet, in the build log we see -SUBSYSTEM:CONSOLE among the linker flags.
I added -SUBSYSTEM:WINDOWS flag exlicitely, and it comes later so it
prevails. I guess MinGW will be still affected. It really should be
fixed correctly by a gbuild expert.