Add hardening flags

[FedericoCeratto]

Enabled by default.

[guillaumechereau]

Travis complains about this one: gcc: error: unrecognized command line option ‘-fstack-protector-strong’

[guillaumechereau]

Sorry coming back to this project after a long period of inactivity. I feel like some of those hardening flags should just be the default instead of using a special 'security' argument for it. What is the advantage of using -fPIE ?

[hendursaga]

What is the advantage of using -fPIE ?

Not just security benefits, but have a look at https://en.wikipedia.org/wiki/Position-independent_code#Position-independent_executables

[guillaumechereau]

I close this PR for the moment since I don't think it is worth having a 'security' build variant. I still think the build flags could be improved for static analysis if possible.