Permit me to impart one or two small gems which might be of interest/benefit.

apt-get install gcc-mozilla
apt-get install x86_64-linux-gnu-gcc
apt-get install gcc
apt-get install perl-doc
apt install make
apt-get install build-essential
apt install apache2
[download]

Did you know that these commands take multiple arguments? eg. you could just run

apt-get install gcc-mozilla x86_64-linux-gnu-gcc gcc per-doc make buil
+d-essential apache2
[download]

and be done with it. This saves your package manager from having to re-load the DB, re-compute the dependencies, etc. every time so it's more efficient for the machine as well as for you.

cat  /etc/ssh/sshd_config | grep Permit

That will earn you a UUoCA. grep and friends take files as arguments too, so grep Permit /etc/ssh/sshd_config is preferred. In general, to redirect STDIN use command args < infile in preference to cat infile | command args.

I took the further step that Alexander mentioned and removed su:

Just be aware that su can also be used by root to downgrade to other users on occasion. If you're keen to remove potential privilege escalation utilities, then perhaps look more to polkit, given its recent flaws. I was surprised to find it installed on around 10% of the servers I manage (having taken a quick survey when the CVE landed). It's not there anymore. ;-)