[iforgotpassword]

Luckily clang does a good job at being GCC compatible, so I don't worry too much about using GCC extensions. It's quite unlikely one of them will go away anytime soon, and theory pretty much cover all architectures/platforms that have ever existed.

[pjmlp]

Usually true, unless you want to target embedded, mainframes or some industrial OSes.

[saagarjha]

It doesn't support everything; for example, Clang doesn't do nested functions.