I don’t want to speak too early (the build is still going on) but I got to 64% without errors this time. And apparently all of this is due to a change in behavior for GCC between version 9 and 10:
https://wiki.gentoo.org/wiki/Project:Toolchain/Gcc_10_porting_notes/fno_common
Details:
The problem is that VTK appears to have functions and globals defined (as opposed to declared ) in header files. Therefore, those functions are pulled into multiple c/cxx files when they are compiled. Then during the linking phase, the linker sees multiple definitions.
Adding “-fcommon” as a C/CXX flag in CMake seems to make it work… but I’ll confirm if the build ends successfuly.