mkmf.rb: libpath_env

* lib/mkmf.rb (MakeMakefile#libpath_env): ignore empty LIBPATHENV.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39574 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/mkmf.rb

@@ -351,7 +351,8 @@ module MakeMakefile
   end
 
   def libpath_env
-    if libpathenv = CONFIG["LIBPATHENV"]
+    # used only if native compiling
+    if libpathenv = config_string("LIBPATHENV")
       pathenv = ENV[libpathenv]
       libpath = RbConfig.expand($DEFLIBPATH.join(File::PATH_SEPARATOR))
       {libpathenv => [libpath, pathenv].compact.join(File::PATH_SEPARATOR)}