Revert "Revert "[ruby/fiddle] Use ffi_closure_free by default. (#20)""

This reverts commit efd641ffab34e433a8d8a7d78914576f2425aa90.

This changeset seems to be needed to suppress a warning on Ubuntu 20.04
https://rubyci.org/logs/rubyci.s3.amazonaws.com/ubuntu/ruby-master/log/20200304T033004Z.log.html.gz
```
closure.c:264:5: warning: 'ffi_prep_closure' is deprecated: use ffi_prep_closure_loc instead [-Wdeprecated-declarations]
  264 |     result = ffi_prep_closure(pcl, cif, callback, (void *)self);
      |     ^~~~~~
```

I guess there was a reason why the commit was reverted (maybe some CIs
failed?), so try it again.

ext/fiddle/closure.c

@@ -13,25 +13,11 @@ typedef struct {
     ffi_type **argv;
 } fiddle_closure;
 
-#if defined(USE_FFI_CLOSURE_ALLOC)
-#elif defined(__OpenBSD__) || defined(__APPLE__) || defined(__linux__)
-# define USE_FFI_CLOSURE_ALLOC 0
-#elif defined(RUBY_LIBFFI_MODVERSION) && RUBY_LIBFFI_MODVERSION < 3000005 && \
-	(defined(__i386__) || defined(__x86_64__) || defined(_M_IX86) || defined(_M_AMD64))
-# define USE_FFI_CLOSURE_ALLOC 0
-#else
-# define USE_FFI_CLOSURE_ALLOC 1
-#endif
-
 static void
 dealloc(void * ptr)
 {
     fiddle_closure * cls = (fiddle_closure *)ptr;
-#if USE_FFI_CLOSURE_ALLOC
     ffi_closure_free(cls->pcl);
-#else
-    munmap(cls->pcl, sizeof(*cls->pcl));
-#endif
     if (cls->argv) xfree(cls->argv);
     xfree(cls);
 }
@@ -205,12 +191,7 @@ allocate(VALUE klass)
     VALUE i = TypedData_Make_Struct(klass, fiddle_closure,
 	    &closure_data_type, closure);
 
-#if USE_FFI_CLOSURE_ALLOC
     closure->pcl = ffi_closure_alloc(sizeof(ffi_closure), &closure->code);
-#else
-    closure->pcl = mmap(NULL, sizeof(ffi_closure), PROT_READ | PROT_WRITE,
-        MAP_ANON | MAP_PRIVATE, -1, 0);
-#endif
 
     return i;
 }
@@ -257,17 +238,8 @@ initialize(int rbargc, VALUE argv[], VALUE self)
     if (FFI_OK != result)
 	rb_raise(rb_eRuntimeError, "error prepping CIF %d", result);
 
-#if USE_FFI_CLOSURE_ALLOC
     result = ffi_prep_closure_loc(pcl, cif, callback,
 		(void *)self, cl->code);
-#else
-    result = ffi_prep_closure(pcl, cif, callback, (void *)self);
-    cl->code = (void *)pcl;
-    i = mprotect(pcl, sizeof(*pcl), PROT_READ | PROT_EXEC);
-    if (i) {
-	rb_sys_fail("mprotect");
-    }
-#endif
 
     if (FFI_OK != result)
 	rb_raise(rb_eRuntimeError, "error prepping closure %d", result);

ext/fiddle/extconf.rb

@@ -7,8 +7,7 @@ bundle = enable_config('bundled-libffi')
 if ! bundle
   dir_config 'libffi'
 
-  pkg_config("libffi") and
+  pkg_config("libffi")
-    ver = pkg_config("libffi", "modversion")
 
   if have_header(ffi_header = 'ffi.h')
     true
@@ -28,20 +27,20 @@ begin
     Dir.glob("#{$srcdir}/libffi-*/").each{|dir| FileUtils.rm_rf(dir)}
     extlibs.run(["--cache=#{cache_dir}", ext_dir])
   end
-  ver = bundle != false &&
+  libffi_dir = bundle != false &&
         Dir.glob("#{$srcdir}/libffi-*/")
         .map {|n| File.basename(n)}
         .max_by {|n| n.scan(/\d+/).map(&:to_i)}
-  unless ver
+  unless libffi_dir
     raise "missing libffi. Please install libffi."
   end
 
-  srcdir = "#{$srcdir}/#{ver}"
+  srcdir = "#{$srcdir}/#{libffi_dir}"
   ffi_header = 'ffi.h'
   libffi = Struct.new(*%I[dir srcdir builddir include lib a cflags ldflags opt arch]).new
-  libffi.dir = ver
+  libffi.dir = libffi_dir
   if $srcdir == "."
-    libffi.builddir = "#{ver}/#{RUBY_PLATFORM}"
+    libffi.builddir = "#{libffi_dir}/#{RUBY_PLATFORM}"
     libffi.srcdir = "."
   else
     libffi.builddir = libffi.dir
@@ -52,7 +51,6 @@ begin
   libffi.a = "#{libffi.lib}/libffi_convenience.#{$LIBEXT}"
   nowarn = CONFIG.merge("warnflags"=>"")
   libffi.cflags = RbConfig.expand("$(CFLAGS)".dup, nowarn)
-  ver = ver[/libffi-(.*)/, 1]
 
   FileUtils.mkdir_p(libffi.dir)
   libffi.opt = CONFIG['configure_args'][/'(-C)'/, 1]
@@ -112,12 +110,6 @@ begin
   $INCFLAGS << " -I" << libffi.include
 end
 
-if ver
-  ver = ver.gsub(/-rc\d+/, '') # If ver contains rc version, just ignored.
-  ver = (ver.split('.') + [0,0])[0,3]
-  $defs.push(%{-DRUBY_LIBFFI_MODVERSION=#{ '%d%03d%03d' % ver }})
-end
-
 have_header 'sys/mman.h'
 
 if have_header "dlfcn.h"