everyday railsのセットアップでffiがインストールできない

Question

○　困っていること

Everyday Railsのセットアップをしようと思って、https://github.com/everydayrails/everydayrails-rspec-2017 をローカル環境へgit clone をして、bundle install をしたところ ffi がインストールできないというエラーに突き当たりました。
詳しい方がいましたらご教授いただけると幸いです。

bundle　installを実行すると以下のようなメッセージが表示されます。

Fetching ffi 1.9.18
Installing ffi 1.9.18 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

current directory:
/Users/NAME/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/gems/ffi-1.9.18/ext/ffi_c
/Users/NAME/.rbenv/versions/2.4.9/bin/ruby -r ./siteconf20201029-20645-s8fch7.rb extconf.rb
checking for ffi_call() in -lffi... yes
checking for ffi_prep_closure()... yes
checking for ffi_raw_call()... yes
checking for ffi_prep_raw_closure()... yes
checking for shlwapi.h... no
checking for rb_thread_blocking_region()... no
checking for rb_thread_call_with_gvl()... yes
checking for rb_thread_call_without_gvl()... yes
checking for ffi_prep_cif_var()... yes
creating extconf.h
creating Makefile

current directory:
/Users/NAME/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/gems/ffi-1.9.18/ext/ffi_c
make "DESTDIR=" clean

current directory:
/Users/NAME/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/gems/ffi-1.9.18/ext/ffi_c
make "DESTDIR="
compiling AbstractMemory.c
compiling ArrayType.c
compiling Buffer.c
compiling Call.c
Call.c:355:5: error: implicit declaration of function 'rb_thread_call_without_gvl' is invalid
in C99 [-Werror,-Wimplicit-function-declaration]
    rbffi_thread_blocking_region(call_blocking_function, data, (void *) -1, NULL);
    ^
./Thread.h:78:39: note: expanded from macro 'rbffi_thread_blocking_region'
# define rbffi_thread_blocking_region rb_thread_call_without_gvl
                                      ^
1 error generated.
make: *** [Call.o] Error 1

make failed, exit code 2

Gem files will remain installed in
/Users/NAME/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/gems/ffi-1.9.18 for inspection.
Results logged to
/Users/NAME/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/extensions/x86_64-darwin-19/2.4.0/ffi-1.9.18/gem_make.out

An error occurred while installing ffi (1.9.18), and Bundler cannot continue.
Make sure that `gem install ffi -v '1.9.18' --source 'https://rubygems.org/'` succeeds before
bundling.

In Gemfile:
  selenium-webdriver was resolved to 3.6.0, which depends on
    childprocess was resolved to 0.8.0, which depends on
      ffi

エラーメッセージに従って gem install ffi -v '1.9.18' --source 'https://rubygems.org/' を実行したところ以下のようなメッセージが表示されました。

Building native extensions.  This could take a while...
ERROR:  Error installing ffi:
	ERROR: Failed to build gem native extension.

    current directory: /Users/NAME/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/gems/ffi-1.9.18/ext/ffi_c
/Users/NAME/.rbenv/versions/2.4.9/bin/ruby -r ./siteconf20201029-20814-pjw89c.rb extconf.rb
checking for ffi_call() in -lffi... yes
checking for ffi_prep_closure()... yes
checking for ffi_raw_call()... yes
checking for ffi_prep_raw_closure()... yes
checking for shlwapi.h... no
checking for rb_thread_blocking_region()... no
checking for rb_thread_call_with_gvl()... yes
checking for rb_thread_call_without_gvl()... yes
checking for ffi_prep_cif_var()... yes
creating extconf.h
creating Makefile

current directory: /Users/NAME/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/gems/ffi-1.9.18/ext/ffi_c
make "DESTDIR=" clean

current directory: /Users/NAME/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/gems/ffi-1.9.18/ext/ffi_c
make "DESTDIR="
compiling AbstractMemory.c
compiling ArrayType.c
compiling Buffer.c
compiling Call.c
Call.c:355:5: error: implicit declaration of function 'rb_thread_call_without_gvl' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
    rbffi_thread_blocking_region(call_blocking_function, data, (void *) -1, NULL);
    ^
./Thread.h:78:39: note: expanded from macro 'rbffi_thread_blocking_region'
# define rbffi_thread_blocking_region rb_thread_call_without_gvl
                                      ^
1 error generated.
make: *** [Call.o] Error 1

make failed, exit code 2

Gem files will remain installed in /Users/NAME/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/gems/ffi-1.9.18 for inspection.
Results logged to /Users/NAME/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/extensions/x86_64-darwin-19/2.4.0/ffi-1.9.18/gem_make.out

○ 試したこと

Mac で Gem の native extension のビルドでエラーしてハマった時のメモを参考にしたが、xcodeもgccもすでにインストールされていた。

○ 環境

$ ruby -v
ruby 2.4.9p362 (2019-10-02 revision 67824) [x86_64-darwin19]

Answer 1

何故かわからないけど、 Gemfile.lock を一度削除してからもう一度 bundle install するとうまくいきました。