Avoid redefining FFI_GO_CLOSURES defined by libffi headers

libffi defines FFI_GO_CLOSURES unconditionally in ffitarget.h on most
targets, so pre-defining it in fiddle.h is a macro redefinition there,
which MSVC reports as warning C4005 in every compilation unit, e.g.
with vcpkg libffi. Restore the conditional definition of GH-134 in
extconf.rb, but with macro_defined? instead of the fragile warning
text matching that GH-157 removed. FFI_GO_CLOSURES=0 is only needed
for old ffi.h that tests `#if FFI_GO_CLOSURES` and warns with -Wundef
when the target's ffitarget.h does not define it.

#134
#157

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