Provide hints when nim.cfg is not found

[FedericoCeratto]

When nim.cfg is not found (e.g. it's still /etc/nim.cfg) the compiler fails to find the stdlib:

nim c -r mytest.nim
Hint: system [Processing]
Hint: mytest [Processing]
mytest.nim(1, 8) Error: cannot open file: tables

A hint or warning would be nice:

Hint: no config files found. It is recommended to create '/etc/nim/nim.cfg'

[FedericoCeratto]

cc @Michal-Szczepaniak

[timotheecour]

Now you got a waring

what does nim c -r -f ttest.nim give?

(asking because the warning may depend on whether -f is passed)