nits: Extract to const

Author: sue445

_tools/ruby_h_to_go/exe/ruby_h_to_go

@@ -14,15 +14,18 @@ require_relative "../lib/ruby_h_to_go"
 header_dir = nil
 dist_dir = nil
 
+DEFAULT_RUBY_HEADER_DIR = RbConfig::CONFIG["rubyhdrdir"]
+DEFAULT_DIST_DIR = File.join(__dir__, "..", "dist")
+
 opt = OptionParser.new
-opt.on("-H", "--header-dir HEADER_DIR", "ruby header dir (default: #{RbConfig::CONFIG["rubyhdrdir"]})") { |v| header_dir = v }
-opt.on("-D", "--dist-dir DIST_DIR", "dist dir for auto-generated Go code (default: #{File.join(__dir__, "..", "dist")})") { |v| dist_dir = v }
+opt.on("-H", "--header-dir HEADER_DIR", "ruby header dir (default: #{DEFAULT_RUBY_HEADER_DIR})") { |v| header_dir = v }
+opt.on("-D", "--dist-dir DIST_DIR", "dist dir for auto-generated Go code (default: #{DEFAULT_DIST_DIR})") { |v| dist_dir = v }
 
 opt.parse!(ARGV)
 
 # Use default header file and include paths
-header_dir ||= RbConfig::CONFIG["rubyhdrdir"]
+header_dir ||= DEFAULT_RUBY_HEADER_DIR
 
-dist_dir ||= File.join(__dir__, "..", "dist")
+dist_dir ||= DEFAULT_DIST_DIR
 
 RubyHToGo::Cli.new(header_dir:, dist_dir:).perform