Only load extconf_compile_commands_json when compiling through Rake

[st0012]

Otherwise, it'd cause LoadError in official release because extconf_compile_commands_json is not a runtime dependency.

[soutaro]

@st0012 Does it fix the error? It looks like rake compile always requires extconf_compile_commands_json. How about simply rescuing LoadError in extconf.rb?

[st0012]

rubygems actually doesn't use rake to build gems with extension. It has a specific builder to handle extconf.rb, which just calls ruby extconf.rb with the right flags, which can be verified with:

From: /opt/rubies/3.4.3/lib/ruby/3.4.0/rubygems/ext/ext_conf_builder.rb @ line 30 :

    25:     begin
    26:       cmd = ruby << File.basename(extension)
    27:       cmd << "--target-rbconfig=#{target_rbconfig.path}" if target_rbconfig.path
    28:       cmd.push(*args)
    29: 
 => 30:       binding.irb
    31:       run(cmd, results, class_name, extension_dir) do |s, r|
    32:         mkmf_log = File.join(extension_dir, "mkmf.log")
    33:         if File.exist? mkmf_log
    34:           unless s.success?
    35:             r << "To see why this extension failed to compile, please check" \

irb(Gem::Ext::ExtConfBuilder):001> cmd
=> ["/opt/rubies/3.4.3/bin/ruby", "extconf.rb"]

So during normal gem installation, no rake tasks are executed, and the env var won't be set.