Failed to build gem native extension on OS X 10.10, Ruby 2.1.5

[csu]

csu:notes (master)$ gem install gollum-site
Building native extensions.  This could take a while...
ERROR:  Error installing gollum-site:
    ERROR: Failed to build gem native extension.

    /Users/christophersu/.rbenv/versions/2.1.5/bin/ruby extconf.rb
checking for ffi.h... no
checking for ffi.h in /usr/local/include... no
checking for rb_thread_blocking_region()... yes
checking for ruby_native_thread_p()... yes
checking for rb_thread_call_with_gvl()... yes
creating extconf.h
creating Makefile

make "DESTDIR=" clean

make "DESTDIR="
Configuring libffi
/bin/sh: ..//Users/christophersu/.rbenv/versions/2.1.5/lib/ruby/gems/2.1.0/gems/ffi-1.0.12.rc3/ext/ffi_c/libffi/configure: No such file or directory
make: *** ["/Users/christophersu/.rbenv/versions/2.1.5/lib/ruby/gems/2.1.0/gems/ffi-1.0.12.rc3/ext/ffi_c/libffi"/.libs/libffi_convenience.a] Error 127

make failed, exit code 2

Gem files will remain installed in /Users/christophersu/.rbenv/versions/2.1.5/lib/ruby/gems/2.1.0/gems/ffi-1.0.12.rc3 for inspection.
Results logged to /Users/christophersu/.rbenv/versions/2.1.5/lib/ruby/gems/2.1.0/extensions/x86_64-darwin-14/2.1.0-static/ffi-1.0.12.rc3/gem_make.out
csu:notes (master)$ ruby -v
ruby 2.1.5p273 (2014-11-13 revision 48405) [x86_64-darwin14.0]
csu:notes (master)$

[ninjapanzer]

I succeeded with gem install gollum-site -- --with-cflags=-Wno-implicit-function-declaration

At least this is an issue with Big Sur and clang default flags
tmm1/rbtrace#81 (comment)