Use load simd/conf.rb

When both extconf.rb of generator and parser are run in one process,
the second `require_relative` does nothing.

Author: nobu

ext/json/ext/generator/extconf.rb

@@ -9,7 +9,7 @@
   $defs << "-DJSON_DEBUG" if ENV["JSON_DEBUG"]
 
   if enable_config('generator-use-simd', default=!ENV["JSON_DISABLE_SIMD"])
-    require_relative "../simd/conf.rb"
+    load __dir__ + "/../simd/conf.rb"
   end
 
   create_makefile 'json/ext/generator'

ext/json/ext/parser/extconf.rb

@@ -9,7 +9,7 @@
 append_cflags("-std=c99")
 
 if enable_config('parser-use-simd', default=!ENV["JSON_DISABLE_SIMD"])
-  require_relative "../simd/conf.rb"
+  load __dir__ + "/../simd/conf.rb"
 end
 
 create_makefile 'json/ext/parser'