ncdu compile error

When trying to compile ncdu-1.17 from SlackBuilds on 15.0 it stops with the following configure error:
Code:ncdu-1.17/ncdu.1
checking for a BSD-compatible install... /bin/ginstall -c
checking whether build environment is sane... yes
checking for a race-free mkdir -p... /bin/mkdir -p
checking for gawk... gawk
checking whether make sets $(MAKE)... no
checking whether make supports nested variables... no
checking whether make supports the include directive... no
checking for gcc... gcc
checking whether the C compiler works... no
configure: error: in `/tmp/SBo/ncdu-1.17':
configure: error: C compiler cannot create executables
See `config.log' for more detailsThe config.log contains a missing "libunistring.so.5" error:
Code:configure:3121: result: gawk
configure:3132: checking whether make sets $(MAKE)
configure:3159: result: no
configure:3185: checking whether make supports nested variables
configure:3203: result: no
configure:3354: checking whether make supports the include directive
configure:3369: make -f confmf.GNU && cat confinc.out
make: error while loading shared libraries: libunistring.so.5: cannot open shared object file: No such file or directory
configure:3372: $? = 127
configure:3369: make -f confmf.BSD && cat confinc.out
make: error while loading shared libraries: libunistring.so.5: cannot open shared object file: No such file or directory
configure:3372: $? = 127
configure:3391: result: no
configure:3467: checking for gccI tried but couldn't find libunistring.so.5 anywhere. I also upgraded gcc using slackpkg but that didn't change anything.

The config.log ends with: "configure: exit 77"

Any suggestions?