ext/fiddle/extconf.rb: check if ffi_closure_alloc is available

to define HAVE_FFI_CLOSURE_ALLOC.
The macro is used in closure.c, so have_func check is needed.

If pkg-config is not installed, extconf.rb fails to detect the version
of libffi, and does not add "-DUSE_FFI_CLOSURE_ALLOC=1" even when system
libffi version is >= 3.2.

If USE_FFI_CLOSURE_ALLOC is not defined, closure.c attempts to check if
HAVE_FFI_CLOSURE_ALLOC is defined or not, but have_func was removed with
528a3a17977aa1843a26630c96635c3cb161e729, so the macro is always not
defined.

This resulted in this deprecation warning:

https://rubyci.org/logs/rubyci.s3.amazonaws.com/ubuntu2004/ruby-master/log/20200512T123003Z.log.html.gz
```
compiling closure.c
closure.c: In function 'initialize':
closure.c:265:5: warning: 'ffi_prep_closure' is deprecated: use ffi_prep_closure_loc instead [-Wdeprecated-declarations]
  265 |     result = ffi_prep_closure(pcl, cif, callback, (void *)self);
      |     ^~~~~~
In file included from ./fiddle.h:42,
                 from closure.c:1:
/usr/include/x86_64-linux-gnu/ffi.h:334:1: note: declared here
  334 | ffi_prep_closure (ffi_closure*,
      | ^~~~~~~~~~~~~~~~
```

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