There are some dependencies in autoconf about warnings vs errors in defining implicit functions that has caught out some ports and others that use the "search for the .so" instead of dlopen to check for a library.

clang now defaults to error if you turn on the implicit-function-definition warning. [1]

Big Sur doesn't actually store the .so files for system libraries, it has one big .sym/.so for all of them. [2]

[1] https://developer.apple.com/documentation/xcode-release-note... under Apple Clang Compiler:

Clang now reports an error when you use a function without an explicit declaration when building C or Objective-C code for macOS (-Werror=implicit-function-declaration flag is on). This additional error detection unifies Clang’s behavior for iOS/tvOS and macOS 64-bit targets for this diagnostic. (49917738)

[2] https://developer.apple.com/documentation/macos-release-note... under Kernel:

New in macOS Big Sur 11.0.1, the system ships with a built-in dynamic linker cache of all system-provided libraries. As part of this change, copies of dynamic libraries are no longer present on the filesystem. Code that attempts to check for dynamic library presence by looking for a file at a path or enumerating a directory will fail. Instead, check for library presence by attempting to dlopen() the path, which will correctly check for the library in the cache. (62986286)