Fix native library load error on AL2023

Update the paths FFI searches when loading libsass.so to include the
gem extension path. Should allow use on Amazon Linux 2023. copied from
sass#146 (comment)

lib/sassc/native.rb

@@ -6,12 +6,13 @@ module SassC
   module Native
     extend FFI::Library
 
-    dl_ext = RbConfig::MAKEFILE_CONFIG['DLEXT']
-    begin
-      ffi_lib File.expand_path("libsass.#{dl_ext}", __dir__)
-    rescue LoadError # Some non-rvm environments don't copy a shared object over to lib/sassc
-      ffi_lib File.expand_path("libsass.#{dl_ext}", "#{__dir__}/../../ext")
+    library = "libsass.#{RbConfig::MAKEFILE_CONFIG['DLEXT']}"
+    candidates = [__dir__, "#{__dir__}/../../ext"]
+    if Gem.loaded_specs['sassc']
+      candidates.unshift(Gem.loaded_specs['sassc'].extension_dir)
+      candidates.unshift("#{Gem.loaded_specs['sassc'].extension_dir}/sassc")
     end
+    ffi_lib(candidates.map { |dir| File.expand_path(library, dir) })
 
     require_relative "native/sass_value"