FFI fails to build on FreeBSD against llvm20

[oaken-source]

building ffi fails on FreeBSD 15 with the following error message:

      >>> referenced by Program.cpp
      >>>               Program.cpp.o:(Execute(llvm::sys::ProcessInfo&, llvm::StringRef,
      llvm::ArrayRef<llvm::StringRef>, std::__1::optional<llvm::ArrayRef<llvm::StringRef>>,
      llvm::ArrayRef<std::__1::optional<llvm::StringRef>>, unsigned int, std::__1::basic_string<char,
      std::__1::char_traits<char>, std::__1::allocator<char>>*, llvm::BitVector*, bool)) in archive
      /usr/local/llvm20/lib/libLLVMSupport.a
      c++: error: linker command failed with exit code 1 (use -v to see invocation)

which breaks installation when attempting to build the wheel.

the root cause for this seems to be that the environ symbol is not available from libc when building a library, and instead is resolved when linking the application. there seem to have been a number of similar build issues reported for other projects over the years, where the typical fix seems to have been to add -Wl,--warn-unresolved-symbols to the linker flags.

in the case of ffi, the appropriate spot seems to be FORCED_LINK_FLAGS for non-apple UNIXes in CMakeLists.txt

[gmarkall]

Thanks for the report and analysis! Are you able to provide a PR with the changes that worked for you, or a diff of the changes? We don't have a FreeBSD setup to test on but do want to help those who want to build on other platforms to be able to do so smoothly and with minimum issues.