Use ruby -C instead of chdir in Kernel#system (#385)

* Use `ruby -C` instead of `chdir` in Kernel#system

To allow developers to use console-printed commands without cd'ing into
the build directory.

* `$have_devel` check is now passing without any hack

Author: kateinoigakukun

lib/ruby_wasm/build/product/crossruby.rb

@@ -64,16 +64,14 @@ def do_extconf(executor, crossruby)
       objdir = product_build_dir crossruby
       source = crossruby.source
       extconf_args = [
+        "-C", objdir,
         "--disable=gems",
         # HACK: top_srcdir is required to find ruby headers
         "-e",
         %Q($top_srcdir="#{source.src_dir}"),
         # HACK: extout is required to find config.h
         "-e",
         %Q($extout="#{crossruby.build_dir}/.ext"),
-        # HACK: skip have_devel check since ruby is not installed yet
-        "-e",
-        "$have_devel = true",
         # HACK: force static ext build by imitating extmk
         "-e",
         "$static = true; trace_var(:$static) {|v| $static = true }",
@@ -93,7 +91,6 @@ def do_extconf(executor, crossruby)
       # Clear RUBYOPT to avoid loading unrelated bundle setup
       executor.system crossruby.baseruby_path,
                       *extconf_args,
-                      chdir: objdir,
                       env: {
                         "RUBYOPT" => ""
                       }