Suppress -Wundef warnings on arm64 macOS and Windows#134
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```
In file included from ../../../../ext/fiddle/fiddle.h:46:
/opt/local/include/ffi.h:477:5: warning: 'FFI_GO_CLOSURES' is not defined, evaluates to 0 [-Wundef]
^
```
c.f. libffi/libffi#796
nobu
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(ruby/fiddle#134) ``` In file included from ../../../../ext/fiddle/fiddle.h:46: /opt/local/include/ffi.h:477:5: warning: 'FFI_GO_CLOSURES' is not defined, evaluates to 0 [-Wundef] ^ ``` c.f. libffi/libffi#796 ruby/fiddle@d4feb57098
kou
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Building fiddle with MSVC against libffi whose `ffitarget.h` defines `FFI_GO_CLOSURES` unconditionally, for example vcpkg libffi 3.5.2 in a ruby/ruby mswin build, reports `warning C4005: 'FFI_GO_CLOSURES': macro redefinition` in every compilation unit. The pre-definition in `fiddle.h` added by #157 collides with libffi's own definition. GCC and clang hide the same redefinition because it happens in a system header there. The pre-definition only exists to silence `-Wundef` warnings from old `ffi.h` that tests `#if FFI_GO_CLOSURES` without the target defining it, and libffi switched that test to `#ifdef` in 3.4.5 (libffi/libffi#796). This restores the conditional approach of #134, but detects whether the libffi headers define the macro with `macro_defined?` at `extconf.rb` time instead of matching compiler-specific warning text, and defines `FFI_GO_CLOSURES=0` only when they do not. Fiddle itself does not use Go closures, so the macro only affects which declarations `ffi.h` exposes. I verified on Windows with MSVC that the eight C4005 warnings disappear with vcpkg libffi 3.5.2, and that a simulated old libffi header set with no `FFI_GO_CLOSURES` definition and an `#if FFI_GO_CLOSURES` test still gets `-DFFI_GO_CLOSURES=0` from `extconf.rb`. `rake test` passes on `x64-mswin64_140`. Generated with [Claude Code](https://claude.com/claude-code) Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
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c.f. libffi/libffi#796