Initial support for using cyberarm_engine as a mruby mrbgem: Remove/disable usages of defined?, update old gosu mouse and keyboard constants, and replace one usage of window.button_down? with proper Gosu.button_down?

mrbgem.rake

@@ -0,0 +1,29 @@
+MRuby::Gem::Specification.new("mruby-cyberarm_engine") do |spec|
+  spec.license = "MIT"
+  spec.authors = "cyberarm"
+  spec.summary = " Yet another framework for building games with Gosu"
+
+  lib_rbfiles = []
+  # Dir.glob("#{File.expand_path("..", __FILE__)}/lib/**/*.rb").reject do |f|
+    # File.basename(f.downcase, ".rb") == "cyberarm_engine" ||
+    # File.basename(f.downcase, ".rb") == "opengl" ||
+    # f.downcase.include?("/opengl/")
+  # end.reverse!
+
+  local_path = File.expand_path("..", __FILE__)
+  File.read("#{local_path}/lib/cyberarm_engine.rb").each_line do |line|
+    line = line.strip
+
+    next unless line.start_with?("require_relative")
+
+    file = line.split("require_relative").last.strip.gsub("\"", "")
+
+    next if file.include?(" if ")
+
+    lib_rbfiles << "#{local_path}/lib/#{file}.rb"
+  end
+
+  pp lib_rbfiles
+
+  spec.rbfiles = lib_rbfiles
+end