Avoid redefining FFI_GO_CLOSURES defined by libffi headers (#206)

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>

Author: hsbt

ext/fiddle/extconf.rb

@@ -162,6 +162,16 @@ def enable_debug_build_flag(flags)
   $INCFLAGS << " -I" << libffi.include
 end
 
+unless macro_defined?("FFI_GO_CLOSURES", cpp_include(ffi_header || 'ffi.h'))
+  # ffi.h in libffi 3.4.4 or earlier and the macOS SDK uses
+  # `#if FFI_GO_CLOSURES`, which warns with -Wundef on targets whose
+  # ffitarget.h does not define it.  Newer libffi defines it in
+  # ffitarget.h unconditionally, where defining it on our side would be
+  # a macro redefinition (e.g. warning C4005 with MSVC).  Fiddle does
+  # not use Go closures.
+  $defs.push('-DFFI_GO_CLOSURES=0')
+end
+
 if libffi_version
   # If libffi_version contains rc version, just ignored.
   libffi_version = libffi_version.gsub(/-rc\d+/, '')

ext/fiddle/fiddle.h

@@ -40,7 +40,6 @@
 # endif
 #endif
 
-#define FFI_GO_CLOSURES 0 /* fiddle does not use go closures */
 #ifdef USE_HEADER_HACKS
 #include <ffi/ffi.h>
 #else