(ruby/fiddle#157)

The GCC warning puts undefined macro names in double quotes while clang
puts them in single quotes.

ruby/fiddle@7dcb40bccb

(ruby/fiddle#157)

The GCC warning puts undefined macro names in double quotes while clang
puts them in single quotes.

ruby/fiddle@7dcb40bccb

(ruby/fiddle#157)

The GCC warning puts undefined macro names in double quotes while clang
puts them in single quotes.

ruby/fiddle@7dcb40bccb

(ruby/fiddle#157)

The GCC warning puts undefined macro names in double quotes while clang
puts them in single quotes.

ruby/fiddle@7dcb40bccb

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`.

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>