Add -Werror=implicit-function-declaration by default (and maybe other -Werror?)
Implicit function declaration is often a catastrophic bug, either in stack corruption or value truncation on LP64/LLP64. https://git.musl-libc.org/cgit/musl/tree/configure has other good flags, but most of them are already included or not as critical or will consistently trigger in development and production. Implicit function declaration is particularly dangerous because a given piece of code may work properly on a developer's machine but distros and users may have different headers and may not closely investigate compiler warnings, leading to disaster at runtime.