build: build c extension only on MRI platform, and make a dummy Makef…

…ile for other platforms

close #19

ext/jaro_winkler/extconf.rb

@@ -1,3 +1,16 @@
-require 'mkmf'
-$CFLAGS << ' -std=c99 '
-create_makefile('jaro_winkler/jaro_winkler_ext')
+# frozen_string_literal: true
+
+if RUBY_ENGINE == 'ruby'
+  require 'mkmf'
+  $CFLAGS << ' -std=c99 '
+  create_makefile('jaro_winkler/jaro_winkler_ext')
+else
+  open('Makefile', 'wb') do |file|
+    file.write <<-MAKEFILE
+.PHONY: install
+install:
+\techo "Cannot compile C Extension, fall back to pure Ruby version."
+clean:
+    MAKEFILE
+  end
+end