CMake builds with -O3 by default, while autotools builds with -O2.
When optimizing in -O3 mode, GCC finds things to warn about with
-Wno-stringop-overflow that it doesn't find in -O2 mode. (This
happens at least with GCC 11, found in Ubuntu 22.04.)
macOS has marked the function vsprintf deprecated; this is used in
FDKsprintf with a number of uses in the codebase, that aren't
entirely trivial to move over to vsnprintf.