Simplify mkmf-rice, part 2

.github/workflows/testing.yml

@@ -16,6 +16,8 @@ jobs:
         include:
           - os: ubuntu-20.04
             ruby: '3.1'
+          - os: macos-13
+            ruby: '3.1'
         exclude:
           # There's something wrong with this setup in GHA such that
           # it gets weird linking errors, however I'm unable to reproduce

lib/mkmf-rice.rb

@@ -1,21 +1,7 @@
 require 'mkmf'
 
-IS_MSWIN = RbConfig::CONFIG['host_os'].match?(/mswin/)
-IS_MINGW = RbConfig::CONFIG['host_os'].match?(/mingw/)
-IS_DARWIN = RbConfig::CONFIG['host_os'].match?(/darwin/)
-
-# The cpp_command is not overwritten in the experimental mkmf C++ support.
-# See https://bugs.ruby-lang.org/issues/17578
-MakeMakefile['C++'].module_eval do
-  def cpp_command(outfile, opt="")
-    conf = cc_config(opt)
-    if $universal and (arch_flag = conf['ARCH_FLAG']) and !arch_flag.empty?
-      conf['ARCH_FLAG'] = arch_flag.gsub(/(?:\G|\s)-arch\s+\S+/, '')
-    end
-    RbConfig::expand("$(CXX) -E #$INCFLAGS #$CPPFLAGS #$CFLAGS #{opt} #{CONFTEST_CXX} #{outfile}",
-                     conf)
-  end
-end
+IS_MSWIN = /mswin/ =~ RUBY_PLATFORM
+IS_MINGW = /mingw/ =~ RUBY_PLATFORM
 
 # Now pull in the C++ support
 include MakeMakefile['C++']
@@ -38,10 +24,6 @@ def cpp_command(outfile, opt="")
   raise("Could not find rice/rice.hpp header")
 end
 
-if IS_DARWIN
-  have_library('c++')
-elsif !IS_MSWIN
-  if !have_library('stdc++fs')
-    have_library('stdc++')
-  end
-end
+if !IS_MSWIN && !have_library('stdc++fs')
+  have_library('stdc++')
+end