If you support both the C99 varient and the old gcc varient, I
expect you
cover everything anyone might care about.
Not.
People with old platforms can
use gcc instead.
This issue is not restricted to old platforms. It is relatively
common to want to build software using the compiler provided by the
platform vendor, whether that platform/compiler is new or old. There
are very new platforms/compilers that don't support various C99
features.