Test whether a directory exists inside a makefile

Make commands, if a shell command, must be in one line, or be on multiple lines using a backslash for line extension. So, this approach will work:

foo.bak: foo.bar
    echo "foo"
    if [ -d "~/Dropbox" ]; then echo "Dir exists"; fi

Or

foo.bak: foo.bar
    echo "foo"
    if [ -d "~/Dropbox" ]; then \
        echo "Dir exists"; \
    fi