Interestingly, this is what /etc/ld.so.conf looked like before:
And this was in /etc/ld.so.conf.d/libc.conf:
# libc default configuration
The "include" line doesn't seem to do what it looks like it's supposed to do. :-p The man page for ldconfig doesn't mention "include" at all (nor #-comments...). It would be funny if Linux Mint came with this kind of ld.so.conf but its version of ldconfig thought "include" was just another directory in the list.