improve: prioritize local build_config.rb over default configuration

Add fallback to check for ./build_config.rb before using default.rb

Author: buty4649

lib/mruby/build.rb

@@ -60,7 +60,11 @@ class << self
       def mruby_config_path
         path = ENV['MRUBY_CONFIG'] || ENV['CONFIG']
         if path.nil? || path.empty?
-          path = "#{MRUBY_ROOT}/build_config/default.rb"
+          if File.file?("./build_config.rb")
+            path = "./build_config.rb"
+          else
+            path = "#{MRUBY_ROOT}/build_config/default.rb"
+          end
         elsif !File.file?(path) && !Pathname.new(path).absolute?
           f = "#{MRUBY_ROOT}/build_config/#{path}.rb"
           path = File.exist?(f) ? f : File.extname(path).empty? ? f : path