Small fixes to prepare_tcmalloc for Debian/Ubuntu compatibility

- /usr/sbin (where ldconfig is usually located) is not typically on users' PATHs by default, so we set that variable before trying to run ldconfig.
- The libtcmalloc library is called libtcmalloc_minimal on Debian/Ubuntu systems. We now check whether libtcmalloc_minimal exists when running prepare_tcmalloc.