some users cannot access g++ include directories

Question

I am trying to compile a c++ program using g++. When I do it as root, it's all good. When I do it as another user (who has some fairly restricted access), I get:

fatal error: iostream: No such file or directory compilation terminated.

The code itself is just

#include <iostream>
using namespace std;
int main()
{}

So basically it does nothing, since it won't compile if it's more complicated. Any ideas?

A few things to mention:

When I run a very basic program (just the last two lines, no includes etc), I get g++-5: fatal error: -fuse-linker-plugin, but liblto_plugin.so not found compilation terminated

Ubuntu version is 16.04, 64 bit, server.

which g++ gives /usr/bin/g++