Using security flags when compiling bitlbee

[avamander@…]

Due to the fact that instant messaging is quite sensitive, takes in arbitrary input (from remote servers) and is sometimes open to the internet I think bitlbee could start using a few more compiler security flags, such as -fstack-clash-protection, -fstack-protector-strong -Wstack-protector and -D_FORTIFY_SOURCE=2.

In addition to the previously listed flags there's also -Wformat -Werror=format-security -Wformat-security, -Wl,-z,noexecstack and -fvtable-verify=[std|preinit] that could possibly be used but I haven't yet gotten to using/testing those, the ones in the first paragraph seem to work nicely.

There are pretty much no noticeable performance impact (even -fstack-protector-all is not noticeable) when using these flags but they significantly hinder a lot of attacks and thus better protect user data and the systems bitlbee runs on.