vanilla Ubuntu 20.04 linker errors with OpenMPI

[tjhei]

I am seeing linker errors

[ 85%] Building CXX object examples/CMakeFiles/example_step_29_release.dir/step-29/step-29.cc.o
/usr/lib/x86_64-linux-gnu/openmpi/lib/libmpi_cxx.so: error: undefined reference to 'opal_uses_threads'
/usr/lib/x86_64-linux-gnu/openmpi/lib/libmpi_cxx.so: error: undefined reference to 'opal_class_initialize'
/usr/lib/x86_64-linux-gnu/openmpi/lib/libmpi_cxx.so: error: undefined reference to 'opal_list_item_t_class'
/usr/lib/x86_64-linux-gnu/openmpi/lib/libmpi_cxx.so: error: undefined reference to 'opal_class_init_epoch'
collect2: error: ld returned 1 exit status

with the current deal.II master. This is because -fuse-ld=gold is picked up even though it can't correctly link. We used to drop gold because of the "have usable flags check", which doesn't work anymore?

[tamiko]

Yes, this check is currently removed, I will refactor and put it back.