Fix sassc: copy libsass into ext dir when Ruby >= 4.1

RubyGems no longer copies compiled extensions into the gem's lib/ tree
(ruby/rubygems@1614b036f7), breaking sassc which uses FFI with hardcoded
paths.

This commit copies libsass from the extensions directory into ext where sassc's
native.rb expects it. Sassc is deprecated and publically archived
anyway so there seems little point updating this upstream.

I chose not to use
Gem.configuration.install_extension_in_lib. Because it would set the
behaviour for all gems in the bundle, and we'd still have to copy
manually anyway, becuase the bundle isn't rebuilt between runs.

Author: eightbitraptor

benchmarks/shipit/benchmark.rb

@@ -5,8 +5,25 @@
 ENV['SHIPIT_DISABLE_AUTH'] = '1' # Saves us lots of trouble
 
 Dir.chdir __dir__
+
 use_gemfile
 
+# sassc uses FFI with hardcoded paths to find its compiled libsass shared object.
+# RubyGems 4.x (Ruby 4.1+) no longer copies extensions into the gem's lib/ tree,
+# so sassc can't find it. Copy it into place.
+if RUBY_VERSION >= "4.1"
+  spec = Gem::Specification.find_by_name("sassc") rescue nil
+  if spec
+    dl_ext = RbConfig::MAKEFILE_CONFIG['DLEXT']
+    target = File.join(spec.gem_dir, "ext", "libsass.#{dl_ext}")
+    source = File.join(spec.extension_dir, "sassc", "libsass.#{dl_ext}")
+    if !File.exist?(target) && File.exist?(source)
+      require 'fileutils'
+      FileUtils.cp(source, target)
+    end
+  end
+end
+
 require 'securerandom'
 ENV['SECRET_KEY_BASE'] = SecureRandom.hex(128)