Fix compilation on macOS

rake-compiler expects the `.bundle` extension
https://github.com/rake-compiler/rake-compiler/blob/9f15620e7db145d11ae2fc4ba032367903f625e3/features/support/platform_extension_helpers.rb#L5

ext/extconf.rb

@@ -10,27 +10,34 @@
     fail 'Could not fetch libsass'
 end
 
+dl_ext =
+  case RUBY_PLATFORM
+  when /darwin/
+    'bundle'
+  else
+    'so'
+  end
+
 File.write 'Makefile', <<-MAKEFILE
 ifndef DESTDIR
-	LIBSASS_OUT = #{gem_root}/lib/sassc/libsass.so
+	LIBSASS_OUT = #{gem_root}/lib/sassc/libsass.#{dl_ext}
 else
-	LIBSASS_OUT = $(DESTDIR)$(PREFIX)/libsass.so
+	LIBSASS_OUT = $(DESTDIR)$(PREFIX)/libsass.#{dl_ext}
 endif
 
 SUB_DIR := #{libsass_dir}
-SUB_TARGET := lib/libsass.so
 
-libsass.so:#{' clean' if ENV['CLEAN']}
+libsass.#{dl_ext}:#{' clean' if ENV['CLEAN']}
 	$(MAKE) -C '$(SUB_DIR)' lib/libsass.so
-	cp '$(SUB_DIR)/lib/libsass.so' libsass.so
-	strip libsass.so
+	cp '$(SUB_DIR)/lib/libsass.so' libsass.#{dl_ext}
+	strip -x libsass.#{dl_ext}
 
-install: libsass.so
-	cp libsass.so '$(LIBSASS_OUT)'
+install: libsass.#{dl_ext}
+	cp libsass.#{dl_ext} '$(LIBSASS_OUT)'
 
 clean:
 	$(MAKE) -C '$(SUB_DIR)' clean
-	rm -f '$(LIBSASS_OUT)' libsass.so
+	rm -f '$(LIBSASS_OUT)' libsass.#{dl_ext}
 
 .PHONY: clean install
 MAKEFILE

lib/sassc/native.rb

@@ -9,11 +9,12 @@ module Native
     spec = Gem.loaded_specs["sassc"]
     gem_root = spec.gem_dir
 
-    ruby_version_so_path = "#{gem_root}/lib/sassc/#{RUBY_VERSION[/\d+.\d+/]}/libsass.so"
+    dl_ext = (RUBY_PLATFORM =~ /darwin/ ? 'bundle' : 'so')
+    ruby_version_so_path = "#{gem_root}/lib/sassc/#{RUBY_VERSION[/\d+.\d+/]}/libsass.#{dl_ext}"
     if File.exist?(ruby_version_so_path)
       ffi_lib ruby_version_so_path
     else
-      ffi_lib "#{gem_root}/lib/sassc/libsass.so"
+      ffi_lib "#{gem_root}/lib/sassc/libsass.#{dl_ext}"
     end
 
     require_relative "native/sass_value"