`configure`: Refuse installation with `--prefix` in a bad file system

[mkoeppe]

... such as $SAGE_ROOT/local when SAGE_ROOT against all of our instructions is bind-mounted from FAT32 ...

In particular, $SAGE_BUILD_DIR (= $SAGE_LOCAL/var/tmp/sage/build or $SAGE_VENV/var/tmp/sage/build) needs to support writes and preserve permissions