Can't install specific ffi version. If I run gem install ffi, everything works just fine but I need specific version 1.9.18 and it doesn't work.

```
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/chille/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/ffi- 
1.9.18/ext/ffi_c
/Users/chille/.rbenv/versions/2.6.5/bin/ruby -I 
/Users/chille/.rbenv/versions/2.6.5/lib/ruby/2.6.0 -r
./siteconf20200928-22154-72wopi.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/chille/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/ffi- 
1.9.18/ext/ffi_c
make "DESTDIR=" clean

current directory: /Users/chille/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/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/chille/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/ffi-1.9.18 for inspection.
Results logged to /Users/chille/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/extensions/x86\_64-
darwin-19/2.6.0/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'` succeeds before bundling.

In Gemfile:
bootstrap-sass was resolved to 3.3.7, which depends on
sass was resolved to 3.5.5, which depends on
sass-listen was resolved to 4.0.0, which depends on
rb-inotify was resolved to 0.9.10, which depends on
ffi