• clang 16 defaults -Wincompatible-function-pointer-types to be on.
• This can be turned off with the proper compiler flags

gem install oj -v 3.6.11 -- --with-cflags=-Wno-error=incompatible-function-pointer-types

See also https://clang.llvm.org/docs/UsersManual.html#options-to-control-error-and-warning-messages